Javascript adding zeros to the beginning of a string (max length 4 chars)

Question

var number = 1310;

should be left alone.

var number = 120;

should be changed to \"0120\";

var num         


        

Answer 1

An approach I like is to add 10^N to the number, where N is the number of zeros you want. Treat the resultant number as a string and slice off the zeroth digit. Of course, you'll want to be careful if your input number might be larger than your pad length, but it's still much faster than the loop method:

// You want to pad four places:
>>> var N = Math.pow(10, 4)
>>> var number = 1310
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"1310"
>>> var number = 120
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"0120"
>>> var number = 10
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"0010"
…

etc. You can make this into a function easily enough:

/**
 * Pad a number with leading zeros to "pad" places:
 *
 * @param number: The number to pad
 * @param pad: The maximum number of leading zeros
 */
function padNumber(number, pad) {
    var N = Math.pow(10, pad);
    return number < N ? ("" + (N + number)).slice(1) : "" + number
}