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String: length

The length data property of a String value contains the length of the string in UTF-16 code units.

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Value

A non-negative integer.

Property attributes of `String: length`
Writable	no
Enumerable	no
Configurable	no

Description

This property returns the number of code units in the string. JavaScript uses UTF-16 encoding, where each Unicode character may be encoded as one or two code units, so it's possible for the value returned by length to not match the actual number of Unicode characters in the string. For common scripts like Latin, Cyrillic, wellknown CJK characters, etc., this should not be an issue, but if you are working with certain scripts, such as emojis, mathematical symbols, or obscure Chinese characters, you may need to account for the difference between code units and characters.

The language specification requires strings to have a maximum length of 2⁵³ - 1 elements, which is the upper limit for precise integers. However, a string with this length needs 16384TiB of storage, which cannot fit in any reasonable device's memory, so implementations tend to lower the threshold, which allows the string's length to be conveniently stored in a 32-bit integer.

In V8 (used by Chrome and Node), the maximum length is 2²⁹ - 24 (~1GiB). On 32-bit systems, the maximum length is 2²⁸ - 16 (~512MiB).
In Firefox, the maximum length is 2³⁰ - 2 (~2GiB). Before Firefox 65, the maximum length was 2²⁸ - 1 (~512MiB).
In Safari, the maximum length is 2³¹ - 1 (~4GiB).

For an empty string, length is 0.

The static property String.length is unrelated to the length of strings. It's the arity of the String function (loosely, the number of formal parameters it has), which is 1.

Since length counts code units instead of characters, if you want to get the number of characters, you can first split the string with its iterator, which iterates by characters:

function getCharacterLength(str) {
  // The string iterator that is used here iterates over characters,
  // not mere code units
  return [...str].length;
}

console.log(getCharacterLength("A\uD87E\uDC04Z")); // 3

Examples

Basic usage

const x = "Mozilla";
const empty = "";

console.log(`${x} is ${x.length} code units long`);
// Mozilla is 7 code units long

console.log(`The empty string has a length of ${empty.length}`);
// The empty string has a length of 0

Strings with length not equal to the number of characters

const emoji = "😄";
console.log(emoji.length); // 2
console.log([...emoji].length); // 1
const adlam = "𞤲𞥋𞤣𞤫";
console.log(adlam.length); // 8
console.log([...adlam].length); // 4
const formula = "∀𝑥∈ℝ,𝑥²≥0";
console.log(formula.length); // 11
console.log([...formula].length); // 9

Assigning to length

Because string is a primitive, attempting to assign a value to a string's length property has no observable effect, and will throw in strict mode.

const myString = "bluebells";

myString.length = 4;
console.log(myString); // "bluebells"
console.log(myString.length); // 9

Specifications

Specification
ECMAScript Language Specification # sec-properties-of-string-instances-length

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`length`	1	12	1	3	1	18	4	10.1	1	1.0	4.4	1.0	0.10.0