Create a string of variable length, filled with a repeated character

Question

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).