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

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs 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/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs 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/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

disallow all tabs (no-tabs)

Some style guides don't allow the use of tab characters at all, including within comments.

Rule Details

This rule looks for tabs anywhere inside a file: code, comments or anything else.

Examples of incorrect code for this rule:

var a /t= 2;

/**
* /t/t it's a test function
*/
function test(){}

var x = 1; // /t test

Examples of correct code for this rule:

var a = 2;

/**
* it's a test function
*/
function test(){}

var x = 1; // test

When Not To Use It

If you have established a standard where having tabs is fine.

Compatibility

• JSCS: disallowTabs Source: http://eslint.org/docs/rules/

Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

/*eslint no-param-reassign: "error"*/

function foo(bar) {
    bar = 13;
}

function foo(bar) {
    bar++;
}

Examples of correct code for this rule:

/*eslint no-param-reassign: "error"*/

function foo(bar) {
    var baz = bar;
}

Options

This rule takes one option, an object, with a boolean property "props" and an array "ignorePropertyModificationsFor". "props" is false by default. If "props" is set to true, this rule warns against the modification of parameter properties unless they're included in "ignorePropertyModificationsFor", which is an empty array by default.

props

Examples of correct code for the default { "props": false } option:

/*eslint no-param-reassign: ["error", { "props": false }]*/

function foo(bar) {
    bar.prop = "value";
}

function foo(bar) {
    delete bar.aaa;
}

function foo(bar) {
    bar.aaa++;
}

Examples of incorrect code for the { "props": true } option:

/*eslint no-param-reassign: ["error", { "props": true }]*/

function foo(bar) {
    bar.prop = "value";
}

function foo(bar) {
    delete bar.aaa;
}

function foo(bar) {
    bar.aaa++;
}

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

/*eslint no-param-reassign: ["error", { "props": true, "ignorePropertyModificationsFor": ["bar"] }]*/

function foo(bar) {
    bar.prop = "value";
}

function foo(bar) {
    delete bar.aaa;
}

function foo(bar) {
    bar.aaa++;
}

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments Source: http://eslint.org/docs/rules/