[javascript] Repeat a string in JavaScript a number of times

In Perl I can repeat a character multiple times using the syntax:

$a = "a" x 10; // results in "aaaaaaaaaa"

Is there a simple way to accomplish this in Javascript? I can obviously use a function, but I was wondering if there was any built in approach, or some other clever technique.

this is how you can call a function and get the result by the helps of Array() and join()

function repeatStringNumTimes(str, num) {_x000D_
  // repeat after me_x000D_
  return num > 0 ? Array(num+1).join(str) : "";_x000D_
}_x000D_
_x000D_
console.log(repeatStringNumTimes("a",10))

Right pads with zeros with no arrays or loops. Just uses repeat() using ES6 2015, which has wide support now. Left pads if you switch the concatenation.

function pad(text, maxLength){ 
  var res = text + "0".repeat(maxLength - text.length);
  return res;
}

console.log(pad('hello', 8)); //hello000

Here is an ES6 version

const repeat = (a,n) => Array(n).join(a+"|$|").split("|$|");_x000D_
repeat("A",20).forEach((a,b) => console.log(a,b+1))

In ES2015/ES6 you can use "*".repeat(n)

So just add this to your projects, and your are good to go.

  String.prototype.repeat = String.prototype.repeat || 
    function(n) {
      if (n < 0) throw new RangeError("invalid count value");
      if (n == 0) return "";
      return new Array(n + 1).join(this.toString()) 
    };

An alternative is:

for(var word = ''; word.length < 10; word += 'a'){}

If you need to repeat multiple chars, multiply your conditional:

for(var word = ''; word.length < 10 * 3; word += 'foo'){}

NOTE: You do not have to overshoot by 1 as with word = Array(11).join('a')

String.prototype.repeat = function (n) { n = Math.abs(n) || 1; return Array(n + 1).join(this || ''); };

// console.log("0".repeat(3) , "0".repeat(-3))
// return: "000" "000"

I'm going to expand on @bonbon's answer. His method is an easy way to "append N chars to an existing string", just in case anyone needs to do that. For example since "a google" is a 1 followed by 100 zeros.

for(var google = '1'; google.length < 1 + 100; google += '0'){}_x000D_
document.getElementById('el').innerText = google;

<div>This is "a google":</div>_x000D_
<div id="el"></div>

NOTE: You do have to add the length of the original string to the conditional.

In a new ES6 harmony, you will have native way for doing this with repeat. Also ES6 right now only experimental, this feature is already available in Edge, FF, Chrome and Safari

"abc".repeat(3) // "abcabcabc"

And surely if repeat function is not available you can use old-good Array(n + 1).join("abc")

If you're not opposed to including a library in your project, lodash has a repeat function.

_.repeat('*', 3);
// ? '***

https://lodash.com/docs#repeat

/**  
 * Repeat a string `n`-times (recursive)
 * @param {String} s - The string you want to repeat.
 * @param {Number} n - The times to repeat the string.
 * @param {String} d - A delimiter between each string.
 */

var repeat = function (s, n, d) {
    return --n ? s + (d || "") + repeat(s, n, d) : "" + s;
};

var foo = "foo";
console.log(
    "%s\n%s\n%s\n%s",

    repeat(foo),        // "foo"
    repeat(foo, 2),     // "foofoo"
    repeat(foo, "2"),   // "foofoo"
    repeat(foo, 2, "-") // "foo-foo"
);

Lodash offers a similar functionality as the Javascript repeat() function which is not available in all browers. It is called _.repeat and available since version 3.0.0:

_.repeat('a', 10);

For all browsers

The following function will perform a lot faster than the option suggested in the accepted answer:

var repeat = function(str, count) {
    var array = [];
    for(var i = 0; i < count;)
        array[i++] = str;
    return array.join('');
}

You'd use it like this :

var repeatedString = repeat("a", 10);

To compare the performance of this function with that of the option proposed in the accepted answer, see this Fiddle and this Fiddle for benchmarks.

For moderns browsers only

In modern browsers, you can now do this using String.prototype.repeat method:

var repeatedString = "a".repeat(10);

Read more about this method on MDN.

This option is even faster. Unfortunately, it doesn't work in any version of Internet explorer. The numbers in the table specify the first browser version that fully supports the method:

For repeat a value in my projects i use repeat

For example:

var n = 6;
for (i = 0; i < n; i++) {
    console.log("#".repeat(i+1))
}

but be careful because this method has been added to the ECMAScript 6 specification.

function repeatString(n, string) {
  var repeat = [];
  repeat.length = n + 1;
  return repeat.join(string);
}

repeatString(3,'x'); // => xxx
repeatString(10,''); // => ""

var stringRepeat = function(string, val) {
  var newString = [];
    for(var i = 0; i < val; i++) {
      newString.push(string);
  }
  return newString.join('');
}

var repeatedString = stringRepeat("a", 1);

Here is what I use:

function repeat(str, num) {
        var holder = [];
        for(var i=0; i<num; i++) {
            holder.push(str);
        }
        return holder.join('');
    }

Convenient if you repeat yourself a lot:

String.prototype.repeat = String.prototype.repeat || function(n){_x000D_
  n= n || 1;_x000D_
  return Array(n+1).join(this);_x000D_
}_x000D_
_x000D_
alert(  'Are we there yet?\nNo.\n'.repeat(10)  )

Another interesting way to quickly repeat n character is to use idea from quick exponentiation algorithm:

var repeatString = function(string, n) {
    var result = '', i;

    for (i = 1; i <= n; i *= 2) {
        if ((n & i) === i) {
            result += string;
        }
        string = string + string;
    }

    return result;
};

The most performance-wice way is https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat

Short version is below.

  String.prototype.repeat = function(count) {
    if (count < 1) return '';
    var result = '', pattern = this.valueOf();
    while (count > 1) {
      if (count & 1) result += pattern;
      count >>>= 1, pattern += pattern;
    }
    return result + pattern;
  };
  var a = "a";
  console.debug(a.repeat(10));

Polyfill from Mozilla:

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;
  }
}

Array(10).fill('a').join('')

Although the most voted answer is a bit more compact, with this approach you don't have to add an extra array item.

Can be used as a one-liner too:

function repeat(str, len) {
    while (str.length < len) str += str.substr(0, len-str.length);
    return str;
}

In CoffeeScript:

( 'a' for dot in [0..10]).join('')