string.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

/** 
 * str: String
 * count: Number
 */

let resultString = str.repeat(count);

Parameters

count

An integer between 0 and +∞: [0, +∞), indicating the number of times to repeat the string in the newly-created string that is to be returned.

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

'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;
    count = +count;
    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 rpt = '';
    for (;;) {
      if ((count & 1) == 1) {
        rpt += str;
      }
      count >>>= 1;
      if (count == 0) {
        break;
      }
      str += str;
    }
    // Could we try:
    // return Array(count + 1).join(this);
    return rpt;
  }
}

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'String.prototype.repeat' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of 'String.prototype.repeat' in that specification.	Draft

Browser compatibility