String.prototype.repeat()

The repeat() method constructs and returns a new string which contains the specified number of copies of the string on which it was called, concatenated together.

Syntax

repeat(count)

Parameters

count
An integer between 0 and +Infinity, indicating the number of times to repeat the string.

Return value

A new string containing the specified number of copies of the given string.

Exceptions

  • RangeError: repeat count must be non-negative.
  • RangeError: repeat count must be less than infinity and not overflow maximum string size.

Examples

Using repeat()

'abc'.repeat(-1)    // RangeError
'abc'.repeat(0)     // ''
'abc'.repeat(1)     // 'abc'
'abc'.repeat(2)     // 'abcabc'
'abc'.repeat(3.5)   // 'abcabcabc' (count will be converted to integer)
'abc'.repeat(1/0)   // RangeError

({ toString: () => 'abc', repeat: String.prototype.repeat }).repeat(2)
// 'abcabc' (repeat() is a generic method)

Polyfill

This method has been added to the ECMAScript 2015 specification and may not be available in all JavaScript implementations yet. However, you can polyfill String.prototype.repeat() with the following snippet:

if (!String.prototype.repeat) {
  String.prototype.repeat = function(count) {
    'use strict';
    if (this == null)
      throw new TypeError('can\'t convert ' + this + ' to object');

    var str = '' + this;
    // To convert string to integer.
    count = +count;
    // Check NaN
    if (count != count)
      count = 0;

    if (count < 0)
      throw new RangeError('repeat count must be non-negative');

    if (count == Infinity)
      throw new RangeError('repeat count must be less than infinity');

    count = Math.floor(count);
    if (str.length == 0 || count == 0)
      return '';

    // Ensuring count is a 31-bit integer allows us to heavily optimize the
    // main part. But anyway, most current (August 2014) browsers can't handle
    // strings 1 << 28 chars or longer, so:
    if (str.length * count >= 1 << 28)
      throw new RangeError('repeat count must not overflow maximum string size');

    var maxCount = str.length * count;
    count = Math.floor(Math.log(count) / Math.log(2));
    while (count) {
       str += str;
       count--;
    }
    str += str.substring(0, maxCount - str.length);
    return str;
  }
}

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'String.prototype.repeat' in that specification.

Browser compatibility

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See also