Repeat Character N Times

问题

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.

回答1:

These days, the repeat string method is implemented almost everywhere. (It is not in Internet Explorer.) So unless you need to support older browsers, you can simply write:

"a".repeat(10)

Before repeat, we used this hack:

Array(11).join("a") // create string with 10 a's: "aaaaaaaaaa"

(Note that an array of length 11 gets you only 10 "a"s, since Array.join puts the argument between the array elements.)

Simon also points out that according to this jsperf, it appears that it's faster in Safari and Chrome (but not Firefox) to repeat a character multiple times by simply appending using a for loop (although a bit less concise).

回答2:

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")

回答3:

Convenient if you repeat yourself a lot:

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

alert(  'Are we there yet?\nNo.\n'.repeat(10)  )

回答4:

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;
  }
}

回答5:

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')

回答6:

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

_.repeat('*', 3);
// → '***

https://lodash.com/docs#repeat

回答7:

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);

回答8:

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.

回答9:

/**  
 * 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"
);

回答10:

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()) 
    };

回答11:

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;
};

回答12:

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.

回答13:

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

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

回答14:

Here is what I use:

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

回答15:

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'){}
document.getElementById('el').innerText = google;

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

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

回答16:

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);

回答17:

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);

回答18:

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;
}

回答19:

In CoffeeScript:

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

回答20:

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"

回答21:

Here is an ES6 version

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

来源：https://stackoverflow.com/questions/1877475/repeat-character-n-times

标签

javascript

string

character

repeat