javascript - Create a string of variable length, filled with a repeated character - Stack Overflow

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So, my question has been asked by someone else in it's Java form here: Java - Create a new String instance with specified length and filled with specific character. Best solution?

. . . but I'm looking for its JavaScript equivalent.

Basically, I'm wanting to dynamically fill text fields with "#" characters, based on the "maxlength" attribute of each field. So, if an input has has maxlength="3", then the field would be filled with "###".

Ideally there would be something like the Java StringUtils.repeat("#", 10);, but, so far, the best option that I can think of is to loop through and append the "#" characters, one at a time, until the max length is reached. I can't shake the feeling that there is a more efficient way to do it than that.

Any ideas?

FYI - I can't simply set a default value in the input, because the "#" characters need to clear on focus, and, if the user didn't enter a value, will need to be "refilled" on blur. It's the "refill" step that I'm concerned with

So, my question has been asked by someone else in it's Java form here: Java - Create a new String instance with specified length and filled with specific character. Best solution?

. . . but I'm looking for its JavaScript equivalent.

Basically, I'm wanting to dynamically fill text fields with "#" characters, based on the "maxlength" attribute of each field. So, if an input has has maxlength="3", then the field would be filled with "###".

Ideally there would be something like the Java StringUtils.repeat("#", 10);, but, so far, the best option that I can think of is to loop through and append the "#" characters, one at a time, until the max length is reached. I can't shake the feeling that there is a more efficient way to do it than that.

Any ideas?

FYI - I can't simply set a default value in the input, because the "#" characters need to clear on focus, and, if the user didn't enter a value, will need to be "refilled" on blur. It's the "refill" step that I'm concerned with

• javascript
• html
• string

Share Improve this question Follow edited May 23, 2017 at 12:10 CommunityBot 111 silver badge asked Jan 15, 2013 at 17:57 talemyntalemyn 7,93044 gold badges3232 silver badges5252 bronze badges 4

• 1 Are you doing this to mask the text of an input field? – Matthew Cox Commented Jan 15, 2013 at 17:58
• @MatthewCox: No, it's more to provide a visual display of maxlength. In this case, it's for a set of phone number fields that are split into the 3 parts of the number. It would show that the first 2 fields need 3 digits and the last needs 4. – talemyn Commented Jan 15, 2013 at 18:00
• possible duplicate of Repeat String - Javascript – Felix Kling Commented Jun 2, 2013 at 17:58
• Today, placeholder is probably recommended. Applying this to all <input>s that have a maxlength looks like document.querySelectorAll("[maxlength]").forEach((element) => element.placeholder = "#".repeat(element.maxLength));. – Sebastian Simon Commented Jan 17, 2022 at 22:12

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12 Answers 12

Sorted by: Reset to default 386

The best way to do this (that I've seen) is

var str = new Array(len + 1).join( character );

That creates an array with the given length, and then joins it with the given string to repeat. The .join() function honors the array length regardless of whether the elements have values assigned, and undefined values are rendered as empty strings.

You have to add 1 to the desired length because the separator string goes between the array elements.

Give this a try :P

s = '#'.repeat(10)

document.body.innerHTML = s

ES2015 the easiest way is to do something like

'X'.repeat(data.length)

X being any string, data.length being the desired length.

see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat

Unfortunately although the Array.join approach mentioned here is terse, it is about 10X slower than a string-concatenation-based implementation. It performs especially badly on large strings. See below for full performance details.

On Firefox, Chrome, Node.js MacOS, Node.js Ubuntu, and Safari, the fastest implementation I tested was:

function repeatChar(count, ch) {
    if (count == 0) {
        return "";
    }
    var count2 = count / 2;
    var result = ch;

    // double the input until it is long enough.
    while (result.length <= count2) {
        result += result;
    }
    // use substring to hit the precise length target without
    // using extra memory
    return result + result.substring(0, count - result.length);
};

This is verbose, so if you want a terse implementation you could go with the naive approach; it still performs betweeb 2X to 10X better than the Array.join approach, and is also faster than the doubling implementation for small inputs. Code:

// naive approach: simply add the letters one by one
function repeatChar(count, ch) {
    var txt = "";
    for (var i = 0; i < count; i++) {
        txt += ch;
    }
    return txt;
}

Further information:

• Run speed test in your own browser
• Full source code of speed test
• Speed test results

I would create a constant string and then call substring on it.

Something like

var hashStore = '########################################';

var Fiveup = hashStore.substring(0,5);

var Tenup = hashStore.substring(0,10);

A bit faster too.

http://jsperf.com/const-vs-join

A great ES6 option would be to padStart an empty string. Like this:

var str = ''.padStart(10, "#");

Note: this won't work in IE (without a polyfill).

Version that works in all browsers

This function does what you want, and performs 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 use it like this :

var repeatedCharacter = 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.

Version for moderns browsers only

In modern browsers, you can now also do this :

var repeatedCharacter = "a".repeat(10) };

This option is even faster. However, 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 Evergreen browsers, this will build a staircase based on an incoming character and the number of stairs to build.
function StairCase(character, input) {
    let i = 0;
    while (i < input) {
        const spaces = " ".repeat(input - (i+1));
        const hashes = character.repeat(i + 1);
        console.log(spaces + hashes);
        i++;
    }
}

//Implement
//Refresh the console
console.clear();
StairCase("#",6);   

You can also add a polyfill for Repeat for older browsers

    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;
      }
    } 

Based on answers from Hogan and Zero Trick Pony. I think this should be both fast and flexible enough to handle well most use cases:

var hash = '####################################################################'

function build_string(length) {  
    if (length == 0) {  
        return ''  
    } else if (hash.length <= length) {  
        return hash.substring(0, length)  
    } else {  
        var result = hash  
        const half_length = length / 2  
        while (result.length <= half_length) {  
            result += result  
        }  
        return result + result.substring(0, length - result.length)  
    }  
}  

You can use the first line of the function as a one-liner if you like:

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

I would do

Buffer.alloc(length, character).toString()

If it's performance you need (prior to ES6), then a combination of substr and a template string is probably best. This function is what I've used for creating space padding strings, but you can change the template to whatever you need:

function strRepeat(intLen, strTemplate) {
  strTemplate = strTemplate || "          ";
  var strTxt = '';
  while(intLen > strTemplate.length) {
    strTxt += strTemplate;
    intLen -= strTemplate.length;
  }
  return ((intLen > 0) ? strTxt + strTemplate.substr(0, intLen) : strTxt);
}

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