How Can JavaScript Pad Strings to a Specific Length?

Can JavaScript Pad Strings to a Given Length?

Padding strings to a desired length is a common need in programming, especially when formatting data for display or storage. JavaScript provides several options for achieving this with varying degrees of complexity and compatibility across different JavaScript platforms.

ES8/ES2017 String.padStart() Method

In 2017, ECMAScript 2017 (ES8) introduced String.padStart() and String.padEnd() methods specifically for padding strings with spaces or other characters. These methods are supported in modern browsers and Node.js versions.

<br>"Jonas".padStart(10); // Default pad string is a space<br>"42".padStart(6, "0"); // Pad with "0"<br>"<em>".padStart(8, "-/|"); // produces '-/|-/|</em>'<br>

For older JavaScript versions, String.padStart() can be implemented using a polyfill.

Pre-ES8 Solutions

Before ES8, several alternative methods were used to pad strings. One common approach was to concatenate the string with a padding string and use slice() to extract the desired portion:

<br>var n = 123</p>
<p>String("00000"   n).slice(-5); // returns 00123<br>("00000"   n).slice(-5); // returns 00123<br>("     "   n).slice(-5); // returns "  123" (with two spaces)<br>

Another technique was to create a string padding extension:

<br>String.prototype.paddingLeft = function (paddingValue) {<br>   return String(paddingValue   this).slice(-paddingValue.length);<br>};<br>

This extension can be used as follows:

<br>function getFormattedTime(date) {<br>  var hours = date.getHours();<br>  var minutes = date.getMinutes();</p>
<p>hours = hours.toString().paddingLeft("00");<br>  minutes = minutes.toString().paddingLeft("00");</p>
<p>return "{0}:{1}".format(hours, minutes);<br>};</p>
<p>String.prototype.format = function () {</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">var args = arguments;
return this.replace(/{(\d+)}/g, function (match, number) {
    return typeof args[number] != 'undefined' ? args[number] : match;
});

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};

This demonstrates string padding using a modified string prototype method.

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