String length - JavaScript 编辑

The length property of a String object contains the length of the string, in UTF-16 code units. length is a read-only data property of string instances.

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Description

This property returns the number of code units in the string. UTF-16, the string format used by JavaScript, uses a single 16-bit code unit to represent the most common characters, but needs to use two code units for less commonly-used characters, so it's possible for the value returned by length to not match the actual number of characters in the string.

ECMAScript 2016 (ed. 7) established a maximum length of 2^53 - 1 elements. Previously, no maximum length was specified. In Firefox, strings have a maximum length of 2**30 - 2 (~1GB). In versions prior to Firefox 65, the maximum length was 2**28 - 1 (~256MB).

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.

Unicode

Since `length` counts code units instead of characters, if you want to get the number of characters you need something like this:

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

// While not recommended, you could add this to each string as follows:

Object.defineProperty(String.prototype, 'charLength', {
  get () {
    return getCharacterLength(this);
  }
});

console.log('A\uD87E\uDC04Z'.charLength); // 3

Examples

Basic usage

let x = 'Mozilla';
let 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);
// expected output: "The empty string has a length of 0"

Assigning to length

let myString = "bluebells";

// Attempting to assign a value to a string's .length property has no observable effect.
myString.length = 4;
console.log(myString);
// expected output: "bluebells"
console.log(myString.length);
// expected output: 9

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• JavaScript String.length and Internationalizing Web Applications