How to Pad Numbers with Leading Zeros in JavaScript?

Padding Numbers with Leading Zeros in JavaScript

In JavaScript, it can be necessary to prepend leading zeros to numbers to achieve a string of a specified length. Here are solutions:

Convert Number to String and Pad with Zeros

One approach is to convert the number to a string and then add leading zeros as needed:

function pad(num, size) {
    num = num.toString();
    while (num.length < size) num = "0" + num;
    return num;
}

For example, pad(5, 2) would return "05".

Utilize Placeholder String

Another option is to utilize a placeholder string that contains the desired number of zeros:

function pad(num, size) {
    var s = "000000000" + num;
    return s.substr(s.length-size);
}

In this case, pad(5, 2) would also return "05".

Considerations for Negative Numbers

If negative numbers are involved, you may need to handle them separately. This can be done by stripping the negative sign and re-adding it after padding:

function pad(num, size) {
    if (num < 0) {
        num = -num;
        return "-" + pad(num, size);
    }
    num = num.toString();
    while (num.length < size) num = "0" + num;
    return num;
}

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