Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

enforce a maximum number of statements allowed in function blocks (max-statements)

The max-statements rule allows you to specify the maximum number of statements allowed in a function.

function foo() {
  var bar = 1; // one statement
  var baz = 2; // two statements
  var qux = 3; // three statements
}

Rule Details

This rule enforces a maximum number of statements allowed in function blocks.

Options

This rule has a number or object option:

• "max" (default 10) enforces a maximum number of statements allows in function blocks

Deprecated: The object property maximum is deprecated; please use the object property max instead.

This rule has an object option:

• "ignoreTopLevelFunctions": true ignores top-level functions

max

Examples of incorrect code for this rule with the default { "max": 10 } option:

/*eslint max-statements: ["error", 10]*/
/*eslint-env es6*/

function foo() {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;

  var foo11 = 11; // Too many.
}

let foo = () => {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;

  var foo11 = 11; // Too many.
};

Examples of correct code for this rule with the default { "max": 10 } option:

/*eslint max-statements: ["error", 10]*/
/*eslint-env es6*/

function foo() {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;
  return function () {

    // The number of statements in the inner function does not count toward the
    // statement maximum.

    return 42;
  };
}

let foo = () => {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;
  return function () {

    // The number of statements in the inner function does not count toward the
    // statement maximum.

    return 42;
  };
}

ignoreTopLevelFunctions

Examples of additional correct code for this rule with the { "max": 10 }, { "ignoreTopLevelFunctions": true } options:

/*eslint max-statements: ["error", 10, { "ignoreTopLevelFunctions": true }]*/

function foo() {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;
  var foo11 = 11;
}

Related Rules

• [complexity](complexity.md)
• [max-depth](max-depth.md)
• [max-len](max-len.md)
• [max-nested-callbacks](max-nested-callbacks.md)
• [max-params](max-params.md) Source: http://eslint.org/docs/rules/

Enforce Callback Error Handling (handle-callback-err)

In Node.js, a common pattern for dealing with asynchronous behavior is called the callback pattern. This pattern expects an Error object or null as the first argument of the callback. Forgetting to handle these errors can lead to some really strange behavior in your application.

function loadData (err, data) {
    doSomething(); // forgot to handle error
}

Rule Details

This rule expects that when you're using the callback pattern in Node.js you'll handle the error.

Options

The rule takes a single string option: the name of the error parameter. The default is "err".

Examples of incorrect code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    doSomething();
}

Examples of correct code for this rule with the default "err" parameter name:

/*eslint handle-callback-err: "error"*/

function loadData (err, data) {
    if (err) {
        console.log(err.stack);
    }
    doSomething();
}

function generateError (err) {
    if (err) {}
}

Examples of correct code for this rule with a sample "error" parameter name:

/*eslint handle-callback-err: ["error", "error"]*/

function loadData (error, data) {
    if (error) {
       console.log(error.stack);
    }
    doSomething();
}

regular expression

Sometimes (especially in big projects) the name of the error variable is not consistent across the project, so you need a more flexible configuration to ensure that the rule reports all unhandled errors.

If the configured name of the error variable begins with a ^ it is considered to be a regexp pattern.

• If the option is "^(err|error|anySpecificError)$", the rule reports unhandled errors where the parameter name can be err, error or anySpecificError.
• If the option is "^.+Error$", the rule reports unhandled errors where the parameter name ends with Error (for example, connectionError or validationError will match).
• If the option is "^.*(e|E)rr", the rule reports unhandled errors where the parameter name matches any string that contains err or Err (for example, err, error, anyError, some_err will match).

When Not To Use It

There are cases where it may be safe for your application to ignore errors, however only ignore errors if you are confident that some other form of monitoring will help you catch the problem.

Further Reading

• The Art Of Node: Callbacks
• Nodejitsu: What are the error conventions? Source: http://eslint.org/docs/rules/

Enforce require() on the top-level module scope (global-require)

In Node.js, module dependencies are included using the require() function, such as:

var fs = require("fs");

While require() may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:

function foo() {

    if (condition) {
        var fs = require("fs");
    }
}

Since require() does a synchronous load, it can cause performance problems when used in other locations.

Further, ES6 modules mandate that import and export statements can only occur in the top level of the module's body.

Rule Details

This rule requires all calls to require() to be at the top level of the module, similar to ES6 import and export statements, which also can occur only at the top level.

Examples of incorrect code for this rule:

/*eslint global-require: "error"*/
/*eslint-env es6*/

// calling require() inside of a function is not allowed
function readFile(filename, callback) {
    var fs = require('fs');
    fs.readFile(filename, callback)
}

// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }

// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }

// you may not require() inside an arrow function body
var getModule = (name) => require(name);

// you may not require() inside of a function body as well
function getModule(name) { return require(name); }

// you may not require() inside of a try/catch block
try {
    require(unsafeModule);
} catch(e) {
    console.log(e);
}

Examples of correct code for this rule:

/*eslint global-require: "error"*/

// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();

// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
    fs.readFile(filename, callback)
}

// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');

// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
    z = require("z");

When Not To Use It

If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require() an optional dependency inside of a try/catch, you can disable this rule for just that dependency using the // eslint-disable-line global-require comment. Source: http://eslint.org/docs/rules/

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 71.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 71.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 63.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 63.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 49.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 49.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 48.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76