Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Disallow unnecessary function binding (no-extra-bind)

The bind() method is used to create functions with specific this values and, optionally, binds arguments to specific values. When used to specify the value of this, it's important that the function actually use this in its function body. For example:

var boundGetName = (function getName() {
    return this.name;
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

This code is an example of a good use of bind() for setting the value of this.

Sometimes during the course of code maintenance, the this value is removed from the function body. In that case, you can end up with a call to bind() that doesn't accomplish anything:

// useless bind
var boundGetName = (function getName() {
    return "ESLint";
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

In this code, the reference to this has been removed but bind() is still used. In this case, the bind() is unnecessary overhead (and a performance hit) and can be safely removed.

Rule Details

This rule is aimed at avoiding the unnecessary use of bind() and as such will warn whenever an immediately-invoked function expression (IIFE) is using bind() and doesn't have an appropriate this value. This rule won't flag usage of bind() that includes function argument binding.

Note: Arrow functions can never have their this value set using bind(). This rule flags all uses of bind() with arrow functions as a problem

Examples of incorrect code for this rule:

/*eslint no-extra-bind: "error"*/
/*eslint-env es6*/

var x = function () {
    foo();
}.bind(bar);

var x = (() => {
    foo();
}).bind(bar);

var x = (() => {
    this.foo();
}).bind(bar);

var x = function () {
    (function () {
      this.foo();
    }());
}.bind(bar);

var x = function () {
    function foo() {
      this.bar();
    }
}.bind(baz);

Examples of correct code for this rule:

/*eslint no-extra-bind: "error"*/

var x = function () {
    this.foo();
}.bind(bar);

var x = function (a) {
    return a + 1;
}.bind(foo, bar);

When Not To Use It

If you are not concerned about unnecessary calls to bind(), you can safely disable this rule.

Further Reading

• Function.prototype.bind
• Understanding JavaScript's Function.prototype.bind Source: http://eslint.org/docs/rules/

Disallow unnecessary .call() and .apply(). (no-useless-call)

The function invocation can be written by Function.prototype.call() and Function.prototype.apply(). But Function.prototype.call() and Function.prototype.apply() are slower than the normal function invocation.

Rule Details

This rule is aimed to flag usage of Function.prototype.call() and Function.prototype.apply() that can be replaced with the normal function invocation.

Examples of incorrect code for this rule:

/*eslint no-useless-call: "error"*/

// These are same as `foo(1, 2, 3);`
foo.call(undefined, 1, 2, 3);
foo.apply(undefined, [1, 2, 3]);
foo.call(null, 1, 2, 3);
foo.apply(null, [1, 2, 3]);

// These are same as `obj.foo(1, 2, 3);`
obj.foo.call(obj, 1, 2, 3);
obj.foo.apply(obj, [1, 2, 3]);

Examples of correct code for this rule:

/*eslint no-useless-call: "error"*/

// The `this` binding is different.
foo.call(obj, 1, 2, 3);
foo.apply(obj, [1, 2, 3]);
obj.foo.call(null, 1, 2, 3);
obj.foo.apply(null, [1, 2, 3]);
obj.foo.call(otherObj, 1, 2, 3);
obj.foo.apply(otherObj, [1, 2, 3]);

// The argument list is variadic.
foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

Known Limitations

This rule compares code statically to check whether or not thisArg is changed. So if the code about thisArg is a dynamic expression, this rule cannot judge correctly.

Examples of incorrect code for this rule:

/*eslint no-useless-call: "error"*/

a[i++].foo.call(a[i++], 1, 2, 3);

Examples of correct code for this rule:

/*eslint no-useless-call: "error"*/

a[++i].foo.call(a[i], 1, 2, 3);

When Not To Use It

If you don't want to be notified about unnecessary .call() and .apply(), you can safely disable this rule. Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Disallow process.exit() (no-process-exit)

The process.exit() method in Node.js is used to immediately stop the Node.js process and exit. This is a dangerous operation because it can occur in any method at any point in time, potentially stopping a Node.js application completely when an error occurs. For example:

if (somethingBadHappened) {
    console.error("Something bad happened!");
    process.exit(1);
}

This code could appear in any module and will stop the entire application when somethingBadHappened is truthy. This doesn't give the application any chance to respond to the error. It's usually better to throw an error and allow the application to handle it appropriately:

if (somethingBadHappened) {
    throw new Error("Something bad happened!");
}

By throwing an error in this way, other parts of the application have an opportunity to handle the error rather than stopping the application altogether. If the error bubbles all the way up to the process without being handled, then the process will exit and a non-zero exit code will returned, so the end result is the same.

Rule Details

This rule aims to prevent the use of process.exit() in Node.js JavaScript. As such, it warns whenever process.exit() is found in code.

Examples of incorrect code for this rule:

/*eslint no-process-exit: "error"*/

process.exit(1);
process.exit(0);

Examples of correct code for this rule:

/*eslint no-process-exit: "error"*/

Process.exit();
var exit = process.exit;

When Not To Use It

There may be a part of a Node.js application that is responsible for determining the correct exit code to return upon exiting. In that case, you should turn this rule off to allow proper handling of the exit code. Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Disallow unnecessary function binding (no-extra-bind)

The bind() method is used to create functions with specific this values and, optionally, binds arguments to specific values. When used to specify the value of this, it's important that the function actually use this in its function body. For example:

var boundGetName = (function getName() {
    return this.name;
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

This code is an example of a good use of bind() for setting the value of this.

Sometimes during the course of code maintenance, the this value is removed from the function body. In that case, you can end up with a call to bind() that doesn't accomplish anything:

// useless bind
var boundGetName = (function getName() {
    return "ESLint";
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

In this code, the reference to this has been removed but bind() is still used. In this case, the bind() is unnecessary overhead (and a performance hit) and can be safely removed.

Rule Details

This rule is aimed at avoiding the unnecessary use of bind() and as such will warn whenever an immediately-invoked function expression (IIFE) is using bind() and doesn't have an appropriate this value. This rule won't flag usage of bind() that includes function argument binding.

Note: Arrow functions can never have their this value set using bind(). This rule flags all uses of bind() with arrow functions as a problem

Examples of incorrect code for this rule:

/*eslint no-extra-bind: "error"*/
/*eslint-env es6*/

var x = function () {
    foo();
}.bind(bar);

var x = (() => {
    foo();
}).bind(bar);

var x = (() => {
    this.foo();
}).bind(bar);

var x = function () {
    (function () {
      this.foo();
    }());
}.bind(bar);

var x = function () {
    function foo() {
      this.bar();
    }
}.bind(baz);

Examples of correct code for this rule:

/*eslint no-extra-bind: "error"*/

var x = function () {
    this.foo();
}.bind(bar);

var x = function (a) {
    return a + 1;
}.bind(foo, bar);

When Not To Use It

If you are not concerned about unnecessary calls to bind(), you can safely disable this rule.

Further Reading

• Function.prototype.bind
• Understanding JavaScript's Function.prototype.bind Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

disallow irregular whitespace (no-irregular-whitespace)

Invalid or irregular whitespace causes issues with ECMAScript 5 parsers and also makes code harder to debug in a similar nature to mixed tabs and spaces.

Various whitespace characters can be inputted by programmers by mistake for example from copying or keyboard shortcuts. Pressing Alt + Space on OS X adds in a non breaking space character for example.

Known issues these spaces cause:

• Zero Width Space
  • Is NOT considered a separator for tokens and is often parsed as an Unexpected token ILLEGAL
  • Is NOT shown in modern browsers making code repository software expected to resolve the visualisation
• Line Separator
  • Is NOT a valid character within JSON which would cause parse errors

Rule Details

This rule is aimed at catching invalid whitespace that is not a normal tab and space. Some of these characters may cause issues in modern browsers and others will be a debugging issue to spot.

This rule disallows the following characters except where the options allow:

\u000B - Line Tabulation (\v) - <vt>
\u000C - Form Feed (\f) - <ff>
\u00A0 - No-Break Space - <nbsp>
\u0085 - Next Line
\u1680 - Ogham Space Mark
\u180E - Mongolian Vowel Separator - <mvs>
\ufeff - Zero Width No-Break Space - <bom>
\u2000 - En Quad
\u2001 - Em Quad
\u2002 - En Space - <ensp>
\u2003 - Em Space - <emsp>
\u2004 - Tree-Per-Em
\u2005 - Four-Per-Em
\u2006 - Six-Per-Em
\u2007 - Figure Space
\u2008 - Punctuation Space - <puncsp>
\u2009 - Thin Space
\u200A - Hair Space
\u200B - Zero Width Space - <zwsp>
\u2028 - Line Separator
\u2029 - Paragraph Separator
\u202F - Narrow No-Break Space
\u205f - Medium Mathematical Space
\u3000 - Ideographic Space</zwsp></puncsp></emsp></ensp></bom></mvs></nbsp></ff></vt>

Options

This rule has an object option for exceptions:

• "skipStrings": true (default) allows any whitespace characters in string literals
• "skipComments": true allows any whitespace characters in comments
• "skipRegExps": true allows any whitespace characters in regular expression literals
• "skipTemplates": true allows any whitespace characters in template literals

skipStrings

Examples of incorrect code for this rule with the default { "skipStrings": true } option:

/*eslint no-irregular-whitespace: "error"*/

function thing() /*<nbsp>*/{
    return 'test';
}

function thing( /*<nbsp>*/){
    return 'test';
}

function thing /*<nbsp>*/(){
    return 'test';
}

function thing᠎/*<mvs>*/(){
    return 'test';
}

function thing() {
    return 'test'; /*<ensp>*/
}

function thing() {
    return 'test'; /*<nbsp>*/
}

function thing() {
    // Description <nbsp>: some descriptive text
}

/*
Description <nbsp>: some descriptive text
*/

function thing() {
    return / <nbsp>regexp/;
}

/*eslint-env es6*/
function thing() {
    return `template <nbsp>string`;
}</nbsp></nbsp></nbsp></nbsp></nbsp></ensp></mvs></nbsp></nbsp></nbsp>

Examples of correct code for this rule with the default { "skipStrings": true } option:

/*eslint no-irregular-whitespace: "error"*/

function thing() {
    return ' <nbsp>thing';
}

function thing() {
    return '​<zwsp>thing';
}

function thing() {
    return 'th <nbsp>ing';
}</nbsp></zwsp></nbsp>

skipComments

Examples of additional correct code for this rule with the { "skipComments": true } option:

/*eslint no-irregular-whitespace: ["error", { "skipComments": true }]*/

function thing() {
    // Description <nbsp>: some descriptive text
}

/*
Description <nbsp>: some descriptive text
*/</nbsp></nbsp>

skipRegExps

Examples of additional correct code for this rule with the { "skipRegExps": true } option:

/*eslint no-irregular-whitespace: ["error", { "skipRegExps": true }]*/

function thing() {
    return / <nbsp>regexp/;
}</nbsp>

skipTemplates

Examples of additional correct code for this rule with the { "skipTemplates": true } option:

/*eslint no-irregular-whitespace: ["error", { "skipTemplates": true }]*/
/*eslint-env es6*/

function thing() {
    return `template <nbsp>string`;
}</nbsp>

When Not To Use It

If you decide that you wish to use whitespace other than tabs and spaces outside of strings in your application.

Further Reading

• ECMA whitespace
• JSON whitespace issues Source: http://eslint.org/docs/rules/

Disallow unnecessary function binding (no-extra-bind)

The bind() method is used to create functions with specific this values and, optionally, binds arguments to specific values. When used to specify the value of this, it's important that the function actually use this in its function body. For example:

var boundGetName = (function getName() {
    return this.name;
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

This code is an example of a good use of bind() for setting the value of this.

Sometimes during the course of code maintenance, the this value is removed from the function body. In that case, you can end up with a call to bind() that doesn't accomplish anything:

// useless bind
var boundGetName = (function getName() {
    return "ESLint";
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

In this code, the reference to this has been removed but bind() is still used. In this case, the bind() is unnecessary overhead (and a performance hit) and can be safely removed.

Rule Details

This rule is aimed at avoiding the unnecessary use of bind() and as such will warn whenever an immediately-invoked function expression (IIFE) is using bind() and doesn't have an appropriate this value. This rule won't flag usage of bind() that includes function argument binding.

Note: Arrow functions can never have their this value set using bind(). This rule flags all uses of bind() with arrow functions as a problem

Examples of incorrect code for this rule:

/*eslint no-extra-bind: "error"*/
/*eslint-env es6*/

var x = function () {
    foo();
}.bind(bar);

var x = (() => {
    foo();
}).bind(bar);

var x = (() => {
    this.foo();
}).bind(bar);

var x = function () {
    (function () {
      this.foo();
    }());
}.bind(bar);

var x = function () {
    function foo() {
      this.bar();
    }
}.bind(baz);

Examples of correct code for this rule:

/*eslint no-extra-bind: "error"*/

var x = function () {
    this.foo();
}.bind(bar);

var x = function (a) {
    return a + 1;
}.bind(foo, bar);

When Not To Use It

If you are not concerned about unnecessary calls to bind(), you can safely disable this rule.

Further Reading

• Function.prototype.bind
• Understanding JavaScript's Function.prototype.bind Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/

Disallow unnecessary function binding (no-extra-bind)

The bind() method is used to create functions with specific this values and, optionally, binds arguments to specific values. When used to specify the value of this, it's important that the function actually use this in its function body. For example:

var boundGetName = (function getName() {
    return this.name;
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

This code is an example of a good use of bind() for setting the value of this.

Sometimes during the course of code maintenance, the this value is removed from the function body. In that case, you can end up with a call to bind() that doesn't accomplish anything:

// useless bind
var boundGetName = (function getName() {
    return "ESLint";
}).bind({ name: "ESLint" });

console.log(boundGetName());      // "ESLint"

In this code, the reference to this has been removed but bind() is still used. In this case, the bind() is unnecessary overhead (and a performance hit) and can be safely removed.

Rule Details

This rule is aimed at avoiding the unnecessary use of bind() and as such will warn whenever an immediately-invoked function expression (IIFE) is using bind() and doesn't have an appropriate this value. This rule won't flag usage of bind() that includes function argument binding.

Note: Arrow functions can never have their this value set using bind(). This rule flags all uses of bind() with arrow functions as a problem

Examples of incorrect code for this rule:

/*eslint no-extra-bind: "error"*/
/*eslint-env es6*/

var x = function () {
    foo();
}.bind(bar);

var x = (() => {
    foo();
}).bind(bar);

var x = (() => {
    this.foo();
}).bind(bar);

var x = function () {
    (function () {
      this.foo();
    }());
}.bind(bar);

var x = function () {
    function foo() {
      this.bar();
    }
}.bind(baz);

Examples of correct code for this rule:

/*eslint no-extra-bind: "error"*/

var x = function () {
    this.foo();
}.bind(bar);

var x = function (a) {
    return a + 1;
}.bind(foo, bar);

When Not To Use It

If you are not concerned about unnecessary calls to bind(), you can safely disable this rule.

Further Reading

• Function.prototype.bind
• Understanding JavaScript's Function.prototype.bind Source: http://eslint.org/docs/rules/

Disallow process.exit() (no-process-exit)

The process.exit() method in Node.js is used to immediately stop the Node.js process and exit. This is a dangerous operation because it can occur in any method at any point in time, potentially stopping a Node.js application completely when an error occurs. For example:

if (somethingBadHappened) {
    console.error("Something bad happened!");
    process.exit(1);
}

This code could appear in any module and will stop the entire application when somethingBadHappened is truthy. This doesn't give the application any chance to respond to the error. It's usually better to throw an error and allow the application to handle it appropriately:

if (somethingBadHappened) {
    throw new Error("Something bad happened!");
}

By throwing an error in this way, other parts of the application have an opportunity to handle the error rather than stopping the application altogether. If the error bubbles all the way up to the process without being handled, then the process will exit and a non-zero exit code will returned, so the end result is the same.

Rule Details

This rule aims to prevent the use of process.exit() in Node.js JavaScript. As such, it warns whenever process.exit() is found in code.

Examples of incorrect code for this rule:

/*eslint no-process-exit: "error"*/

process.exit(1);
process.exit(0);

Examples of correct code for this rule:

/*eslint no-process-exit: "error"*/

Process.exit();
var exit = process.exit;

When Not To Use It

There may be a part of a Node.js application that is responsible for determining the correct exit code to return upon exiting. In that case, you should turn this rule off to allow proper handling of the exit code. Source: http://eslint.org/docs/rules/

Disallow process.exit() (no-process-exit)

The process.exit() method in Node.js is used to immediately stop the Node.js process and exit. This is a dangerous operation because it can occur in any method at any point in time, potentially stopping a Node.js application completely when an error occurs. For example:

if (somethingBadHappened) {
    console.error("Something bad happened!");
    process.exit(1);
}

This code could appear in any module and will stop the entire application when somethingBadHappened is truthy. This doesn't give the application any chance to respond to the error. It's usually better to throw an error and allow the application to handle it appropriately:

if (somethingBadHappened) {
    throw new Error("Something bad happened!");
}

By throwing an error in this way, other parts of the application have an opportunity to handle the error rather than stopping the application altogether. If the error bubbles all the way up to the process without being handled, then the process will exit and a non-zero exit code will returned, so the end result is the same.

Rule Details

This rule aims to prevent the use of process.exit() in Node.js JavaScript. As such, it warns whenever process.exit() is found in code.

Examples of incorrect code for this rule:

/*eslint no-process-exit: "error"*/

process.exit(1);
process.exit(0);

Examples of correct code for this rule:

/*eslint no-process-exit: "error"*/

Process.exit();
var exit = process.exit;

When Not To Use It

There may be a part of a Node.js application that is responsible for determining the correct exit code to return upon exiting. In that case, you should turn this rule off to allow proper handling of the exit code. Source: http://eslint.org/docs/rules/

Enforce Return After Callback (callback-return)

The callback pattern is at the heart of most I/O and event-driven programming in JavaScript.

function doSomething(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

To prevent calling the callback multiple times it is important to return anytime the callback is triggered outside of the main function body. Neglecting this technique often leads to issues where you do something more than once. For example, in the case of an HTTP request, you may try to send HTTP headers more than once leading Node.js to throw a Can't render headers after they are sent to the client. error.

Rule Details

This rule is aimed at ensuring that callbacks used outside of the main function block are always part-of or immediately preceding a return statement. This rule decides what is a callback based on the name of the function being called.

Options

The rule takes a single option - an array of possible callback names - which may include object methods. The default callback names are callback, cb, next.

Default callback names

Examples of incorrect code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err);
    }
    callback();
}

Examples of correct code for this rule with the default ["callback", "cb", "next"] option:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        return callback(err);
    }
    callback();
}

Supplied callback names

Examples of incorrect code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        send.error(err);
    }
    send.success();
}

Examples of correct code for this rule with the option ["done", "send.error", "send.success"]:

/*eslint callback-return: ["error", ["done", "send.error", "send.success"]]*/

function foo(err, done) {
    if (err) {
        return done(err);
    }
    done();
}

function bar(err, send) {
    if (err) {
        return send.error(err);
    }
    send.success();
}

Known Limitations

Because it is difficult to understand the meaning of a program through static analysis, this rule has limitations:

• false negatives when this rule reports correct code, but the program calls the callback more than one time (which is incorrect behavior)
• false positives when this rule reports incorrect code, but the program calls the callback only one time (which is correct behavior)

Passing the callback by reference

The static analysis of this rule does not detect that the program calls the callback if it is an argument of a function (for example, setTimeout).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        setTimeout(callback, 0); // this is bad, but WILL NOT warn
    }
    callback();
}

Triggering the callback within a nested function

The static analysis of this rule does not detect that the program calls the callback from within a nested function or an immediately-invoked function expression (IIFE).

Example of a false negative when this rule reports correct code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        process.nextTick(function() {
            return callback(); // this is bad, but WILL NOT warn
        });
    }
    callback();
}

If/else statements

The static analysis of this rule does not detect that the program calls the callback only one time in each branch of an if statement.

Example of a false positive when this rule reports incorrect code:

/*eslint callback-return: "error"*/

function foo(err, callback) {
    if (err) {
        callback(err); // this is fine, but WILL warn
    } else {
        callback();    // this is fine, but WILL warn
    }
}

When Not To Use It

There are some cases where you might want to call a callback function more than once. In those cases this rule may lead to incorrect behavior. In those cases you may want to reserve a special name for those callbacks and not include that in the list of callbacks that trigger warnings.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions?

Related Rules

• [handle-callback-err](handle-callback-err.md) Source: http://eslint.org/docs/rules/