Checking for an array

JavaScriptJunior~1 min read

1. The modern and reliable approach → `Array.isArray()`

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Array.isArray(value);

This is the standard ES5 method specifically created for this check. It returns true if the value is an array, and false in all other cases.

Examples for `Array.isArray()`

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Array.isArray([1, 2, 3]);     // true
Array.isArray([]);             // true
Array.isArray('text');         // false
Array.isArray({ 0: 'a', 1: 'b', length: 2 }); // false
Array.isArray(null);           // false

It works correctly even across different contexts (for example, iframe, window, and so on). This makes it the only 100% reliable approach.

2. The old approach → `instanceof Array`

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value instanceof Array

Example:

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console.log([1, 2, 3] instanceof Array); // true
console.log({} instanceof Array);        // false

Drawback: if the array was created in a different window or iframe, the check returns false, because it has a different Array constructor.

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// example (pseudocode)
iframe.contentWindow.Array !== window.Array;

3. An alternative (manual) → checking via `Object.prototype.toString`

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Object.prototype.toString.call(value) === '[object Array]'

Example:

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console.log(Object.prototype.toString.call([1, 2, 3])); // "[object Array]"
console.log(Object.prototype.toString.call({}));        // "[object Object]"

Works correctly, but looks cumbersome: nowadays it is almost always replaced with Array.isArray().

Comparing all the approaches

Method	Returns	Pros	Cons
`Array.isArray()`	`true/false`	Modern, reliable	None
`instanceof Array`	`true/false`	Clear syntax	Does not work across different contexts
`Object.prototype.toString.call()`	`"[object Array]"`	Always works	Verbose to write

Examples of checking different values

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console.log(Array.isArray([1, 2, 3]));      // true
console.log(Array.isArray('hello'));        // false
console.log(Array.isArray({ length: 0 }));  // false
console.log(Array.isArray(new Array()));    // true
console.log(Array.isArray(null));           // false

In short

Use Array.isArray(value): it is the most reliable and readable way to check whether a value is an array.

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