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copied!<p>I actually had to come up with something like this recently. I figured there had to be a way to do it without using loops.</p> <p>This is what I came up with.</p> <pre><code>function zeroPad(num, numZeros) { var n = Math.abs(num); var zeros = Math.max(0, numZeros - Math.floor(n).toString().length ); var zeroString = Math.pow(10,zeros).toString().substr(1); if( num < 0 ) { zeroString = '-' + zeroString; } return zeroString+n; } </code></pre> <p>Then just use it providing a number to zero pad:</p> <pre><code>> zeroPad(50,4); "0050" </code></pre> <p>If the number is larger than the padding, the number will expand beyond the padding:</p> <pre><code>> zeroPad(51234, 3); "51234" </code></pre> <p>Decimals are fine too!</p> <pre><code>> zeroPad(51.1234, 4); "0051.1234" </code></pre> <p>If you don't mind polluting the global namespace you can add it to Number directly:</p> <pre><code>Number.prototype.leftZeroPad = function(numZeros) { var n = Math.abs(this); var zeros = Math.max(0, numZeros - Math.floor(n).toString().length ); var zeroString = Math.pow(10,zeros).toString().substr(1); if( this < 0 ) { zeroString = '-' + zeroString; } return zeroString+n; } </code></pre> <p>And if you'd rather have decimals take up space in the padding:</p> <pre><code>Number.prototype.leftZeroPad = function(numZeros) { var n = Math.abs(this); var zeros = Math.max(0, numZeros - n.toString().length ); var zeroString = Math.pow(10,zeros).toString().substr(1); if( this < 0 ) { zeroString = '-' + zeroString; } return zeroString+n; } </code></pre> <p>Cheers!</p> <hr> <hr> <p><a href="https://stackoverflow.com/users/2030088/xdr">XDR</a> came up with <strong>a logarithmic variation</strong> that seems to perform better.</p> <p><strong>WARNING</strong>: This function fails if num equals zero (e.g. zeropad(0, 2))</p> <pre><code>function zeroPad (num, numZeros) { var an = Math.abs (num); var digitCount = 1 + Math.floor (Math.log (an) / Math.LN10); if (digitCount >= numZeros) { return num; } var zeroString = Math.pow (10, numZeros - digitCount).toString ().substr (1); return num < 0 ? '-' + zeroString + an : zeroString + an; } </code></pre> <hr> <p>Speaking of performance, <a href="https://stackoverflow.com/users/1608468/tomsmeding">tomsmeding</a> <a href="http://jsperf.com/left-zero-pad" rel="noreferrer">compared the top 3 answers</a> (<a href="http://jsperf.com/left-zero-pad/10" rel="noreferrer">4 with the log variation</a>). Guess which one <em>majorly</em> outperformed the other two? :)</p>

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