Limit Cyclomatic Complexity (complexity)

Cyclomatic complexity measures the number of linearly independent paths through a program's source code. This rule allows setting a cyclomatic complexity threshold.

function a(x) {
    if (true) {
        return x; // 1st path
    } else if (false) {
        return x+1; // 2nd path
    } else {
        return 4; // 3rd path
    }
}

Rule Details

This rule is aimed at reducing code complexity by capping the amount of cyclomatic complexity allowed in a program. As such, it will warn when the cyclomatic complexity crosses the configured threshold (default is 20).

Examples of incorrect code for a maximum of 2:

/*eslint complexity: ["error", 2]*/

function a(x) {
    if (true) {
        return x;
    } else if (false) {
        return x+1;
    } else {
        return 4; // 3rd path
    }
}

Examples of correct code for a maximum of 2:

/*eslint complexity: ["error", 2]*/

function a(x) {
    if (true) {
        return x;
    } else {
        return 4;
    }
}

Options

Optionally, you may specify a max object property:

"complexity": ["error", 2]

is equivalent to

"complexity": ["error", { "max": 2 }]

Deprecated: the object property maximum is deprecated. Please use the property max instead.

When Not To Use It

If you can't determine an appropriate complexity limit for your code, then it's best to disable this rule.

Further Reading

• About Complexity
• Complexity Analysis of JavaScript Code

Related Rules

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

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

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 Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/

Disallow Extending of Native Objects (no-extend-native)

In JavaScript, you can extend any object, including builtin or "native" objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtins.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

A common suggestion to avoid this problem would be to wrap the inside of the for loop with users.hasOwnProperty(id). However, if this rule is strictly enforced throughout your codebase you won't need to take that step.

Rule Details

Disallows directly modifying the prototype of builtin objects.

Examples of incorrect code for this rule:

/*eslint no-extend-native: "error"*/

Object.prototype.a = "a";
Object.defineProperty(Array.prototype, "times", { value: 999 });

Options

This rule accepts an exceptions option, which can be used to specify a list of builtins for which extensions will be allowed.

exceptions

Examples of correct code for the sample { "exceptions": ["Object"] } option:

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/

Object.prototype.a = "a";

Known Limitations

This rule does not report any of the following less obvious approaches to modify the prototype of builtin objects:

var x = Object;
x.prototype.thing = a;

eval("Array.prototype.forEach = 'muhahaha'");

with(Array) {
    prototype.thing = 'thing';
};

window.Function.prototype.bind = 'tight';

When Not To Use It

You may want to disable this rule when working with polyfills that try to patch older versions of JavaScript with the latest spec, such as those that might Function.prototype.bind or Array.prototype.forEach in a future-friendly way.

Related Rules

• [no-global-assign](no-global-assign.md) Source: http://eslint.org/docs/rules/