Suggest using the rest parameters instead of arguments (prefer-rest-params)

There are rest parameters in ES2015. We can use that feature for variadic functions instead of the arguments variable.

arguments does not have methods of Array.prototype, so it's a bit of an inconvenience.

Rule Details

This rule is aimed to flag usage of arguments variables.

Examples

Examples of incorrect code for this rule:

function foo() {
    console.log(arguments);
}

function foo(action) {
    var args = Array.prototype.slice.call(arguments, 1);
    action.apply(null, args);
}

function foo(action) {
    var args = [].slice.call(arguments, 1);
    action.apply(null, args);
}

Examples of correct code for this rule:

function foo(...args) {
    console.log(args);
}

function foo(action, ...args) {
    action.apply(null, args); // or `action(...args)`, related to the `prefer-spread` rule.
}

// Note: the implicit arguments can be overwritten.
function foo(arguments) {
    console.log(arguments); // This is the first argument.
}
function foo() {
    var arguments = 0;
    console.log(arguments); // This is a local variable.
}

When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don't want to be notified about arguments variables, then it's safe to disable this rule.

Related Rules

• [prefer-spread](prefer-spread.md) Source: http://eslint.org/docs/rules/

Require braces in arrow function body (arrow-body-style)

Arrow functions have two syntactic forms for their function bodies. They may be defined with a block body (denoted by curly braces) () => { ... } or with a single expression () => ..., whose value is implicitly returned.

Rule Details

This rule can enforce or disallow the use of braces around arrow function body.

Options

The rule takes one or two options. The first is a string, which can be:

• "always" enforces braces around the function body
• "as-needed" enforces no braces where they can be omitted (default)
• "never" enforces no braces around the function body (constrains arrow functions to the role of returning an expression)

The second one is an object for more fine-grained configuration when the first option is "as-needed". Currently, the only available option is requireReturnForObjectLiteral, a boolean property. It's false by default. If set to true, it requires braces and an explicit return for object literals.

"arrow-body-style": ["error", "always"]

always

Examples of incorrect code for this rule with the "always" option:

/*eslint arrow-body-style: ["error", "always"]*/
/*eslint-env es6*/
let foo = () => 0;

Examples of correct code for this rule with the "always" option:

let foo = () => {
    return 0;
};
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};

as-needed

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint arrow-body-style: ["error", "as-needed"]*/
/*eslint-env es6*/

let foo = () => {
    return 0;
};
let foo = () => {
    return {
       bar: {
            foo: 1,
            bar: 2,
        }
    };
};

Examples of correct code for this rule with the default "as-needed" option:

/*eslint arrow-body-style: ["error", "as-needed"]*/
/*eslint-env es6*/

let foo = () => 0;
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};
let foo = () => ({
    bar: {
        foo: 1,
        bar: 2,
    }
});
let foo = () => { bar(); };
let foo = () => {};
let foo = () => { /* do nothing */ };
let foo = () => {
    // do nothing.
};
let foo = () => ({ bar: 0 });

requireReturnForObjectLiteral

This option is only applicable when used in conjunction with the "as-needed" option.

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

/*eslint arrow-body-style: ["error", "as-needed", { "requireReturnForObjectLiteral": true }]*/
/*eslint-env es6*/
let foo = () => ({});
let foo = () => ({ bar: 0 });

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

/*eslint arrow-body-style: ["error", "as-needed", { "requireReturnForObjectLiteral": true }]*/
/*eslint-env es6*/

let foo = () => {};
let foo = () => { return { bar: 0 }; };

never

Examples of incorrect code for this rule with the "never" option:

/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => {
    return 0;
};
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};

Examples of correct code for this rule with the "never" option:

/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => 0;
let foo = () => ({ foo: 0 });

Source: http://eslint.org/docs/rules/

require return statements to either always or never specify values (consistent-return)

Unlike statically-typed languages which enforce that a function returns a specified type of value, JavaScript allows different code paths in a function to return different types of values.

A confusing aspect of JavaScript is that a function returns undefined if any of the following are true:

• it does not execute a return statement before it exits
• it executes return which does not specify a value explicitly
• it executes return undefined
• it executes return void followed by an expression (for example, a function call)
• it executes return followed by any other expression which evaluates to undefined

If any code paths in a function return a value explicitly but some code path do not return a value explicitly, it might be a typing mistake, especially in a large function. In the following example:

• a code path through the function returns a Boolean value true
• another code path does not return a value explicitly, therefore returns undefined implicitly

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return;
    }
}

Rule Details

This rule requires return statements to either always or never specify values. This rule ignores function definitions where the name begins with an uppercase letter, because constructors (when invoked with the new operator) return the instantiated object implicitly if they do not return another object explicitly.

Examples of incorrect code for this rule:

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

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return;
    }
}

function doSomething(condition) {
    if (condition) {
        return true;
    }
}

Examples of correct code for this rule:

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

function doSomething(condition) {
    if (condition) {
        return true;
    } else {
        return false;
    }
}

function Foo() {
    if (!(this instanceof Foo)) {
        return new Foo();
    }

    this.a = 0;
}

Options

This rule has an object option:

• "treatUndefinedAsUnspecified": false (default) always either specify values or return undefined implicitly only.
• "treatUndefinedAsUnspecified": true always either specify values or return undefined explicitly or implicitly.

treatUndefinedAsUnspecified

Examples of incorrect code for this rule with the default { "treatUndefinedAsUnspecified": false } option:

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": false }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

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

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    return true;
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    return true;
}

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

/*eslint consistent-return: ["error", { "treatUndefinedAsUnspecified": true }]*/

function foo(callback) {
    if (callback) {
        return void callback();
    }
    // no return statement
}

function bar(condition) {
    if (condition) {
        return undefined;
    }
    // no return statement
}

When Not To Use It

If you want to allow functions to have different return behavior depending on code branching, then it is safe to disable this rule. Source: http://eslint.org/docs/rules/