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    String.prototype.repeat()

    This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal.
    Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

    Summary

    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.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.

    Throws

    RangeError
    The repeat count must be positive and less than infinity.

    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 6 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;
        }
        return rpt;
      }
    }
    

    Specifications

    Specification Status Comment
    ECMAScript 6 (ECMA-262)
    The definition of 'String.prototype.repeat' in that specification.
    Release Candidate Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 41  24 (24) Not supported Not supported Not supported
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support Not supported 36 (behind a flag) 24.0 (24) Not supported Not supported Not supported

     

    Document Tags and Contributors

    Last updated by: ziyunfei,
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