no-magic-numbers

Disallow magic numbers

‘Magic numbers’ are numbers that occur multiple times in code without an explicit meaning. They should preferably be replaced by named constants.

var now = Date.now(),
    inOneHour = now + (60 * 60 * 1000);
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Rule Details

The no-magic-numbers rule aims to make code more readable and refactoring easier by ensuring that special numbers are declared as constants to make their meaning explicit.

Examples of incorrect code for this rule:

/*eslint no-magic-numbers: "error"*/

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * 0.25);
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/*eslint no-magic-numbers: "error"*/

var data = ['foo', 'bar', 'baz'];

var dataLast = data[2];
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/*eslint no-magic-numbers: "error"*/

var SECONDS;

SECONDS = 60;
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Examples of correct code for this rule:

/*eslint no-magic-numbers: "error"*/

var TAX = 0.25;

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * TAX);
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Options

ignore

An array of numbers to ignore. It’s set to [] by default. If provided, it must be an Array.

The array can contain values of number and string types. If it’s a string, the text must be parsed as bigint literal (e.g., "100n").

Examples of correct code for the sample { "ignore": [1] } option:

/*eslint no-magic-numbers: ["error", { "ignore": [1] }]*/

var data = ['foo', 'bar', 'baz'];
var dataLast = data.length && data[data.length - 1];
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Examples of correct code for the sample { "ignore": ["1n"] } option:

/*eslint no-magic-numbers: ["error", { "ignore": ["1n"] }]*/

foo(1n);
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ignoreArrayIndexes

A boolean to specify if numbers used in the context of array indexes (e.g., data[2]) are considered okay. false by default.

This option allows only valid array indexes: numbers that will be coerced to one of "0", "1", "2" … "4294967294".

Arrays are objects, so they can have property names such as "-1" or "2.5". However, those are just “normal” object properties that don’t represent array elements. They don’t influence the array’s length, and they are ignored by array methods like .map or .forEach.

Additionally, since the maximum array length is 2³² - 1, all values above 2³² - 2 also represent just normal property names and are thus not considered to be array indexes.

Examples of correct code for the { "ignoreArrayIndexes": true } option:

/*eslint no-magic-numbers: ["error", { "ignoreArrayIndexes": true }]*/

var item = data[2];

data[100] = a;

f(data[0]);

a = data[-0]; // same as data[0], -0 will be coerced to "0"

a = data[0xAB];

a = data[5.6e1];

a = data[10n]; // same as data[10], 10n will be coerced to "10"

a = data[4294967294]; // max array index
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Examples of incorrect code for the { "ignoreArrayIndexes": true } option:

/*eslint no-magic-numbers: ["error", { "ignoreArrayIndexes": true }]*/

f(2); // not used as array index

a = data[-1];

a = data[2.5];

a = data[5.67e1];

a = data[-10n];

a = data[4294967295]; // above the max array index

a = data[1e500]; // same as data["Infinity"]
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ignoreDefaultValues

A boolean to specify if numbers used in default value assignments are considered okay. false by default.

Examples of correct code for the { "ignoreDefaultValues": true } option:

/*eslint no-magic-numbers: ["error", { "ignoreDefaultValues": true }]*/

const { tax = 0.25 } = accountancy;

function mapParallel(concurrency = 3) { /***/ }
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/*eslint no-magic-numbers: ["error", { "ignoreDefaultValues": true }]*/

let head;
[head = 100] = []
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ignoreClassFieldInitialValues

A boolean to specify if numbers used as initial values of class fields are considered okay. false by default.

Examples of correct code for the { "ignoreClassFieldInitialValues": true } option:

/*eslint no-magic-numbers: ["error", { "ignoreClassFieldInitialValues": true }]*/

class C {
    foo = 2;
    bar = -3;
    #baz = 4;
    static qux = 5;
}
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Examples of incorrect code for the { "ignoreClassFieldInitialValues": true } option:

/*eslint no-magic-numbers: ["error", { "ignoreClassFieldInitialValues": true }]*/

class C {
    foo = 2 + 3;
}

class D {
    2;
}
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enforceConst

A boolean to specify if we should check for the const keyword in variable declaration of numbers. false by default.

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

/*eslint no-magic-numbers: ["error", { "enforceConst": true }]*/

var TAX = 0.25;

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * TAX);
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detectObjects

A boolean to specify if we should detect numbers when setting object properties for example. false by default.

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

/*eslint no-magic-numbers: ["error", { "detectObjects": true }]*/

var magic = {
  tax: 0.25
};

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * magic.tax);
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Examples of correct code for the { "detectObjects": true } option:

/*eslint no-magic-numbers: ["error", { "detectObjects": true }]*/

var TAX = 0.25;

var magic = {
  tax: TAX
};

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * magic.tax);
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Version

This rule was introduced in ESLint v1.7.0.

Resources

• Rule source
• Tests source