How can I format an integer to a specific length in javascript?

Question

I have a number in Javascript, that I know is less than 10000 and also non-negative. I want to display it as a four-digit number, with leading zeroes. Is there anything more elegant than the following?

if(num<10) num="000"+num; else if(num<100) num="00"+num; else if(num<1000) num="0"+num; 

I want something that is built into Javascript, but I can't seem to find anything.

Answer 1

The simplest way I can think of is this:

("000" + num).slice(-4) 

A padded number is a string.
When you add a number to a string, it is converted to a string.
Strings has the method slice, that retuns a fixed length piece of the string.
If length is negative the returned string is sliced from the end of the string.

to test:

var num=12; console.log(("000" + num).slice(-4)); // Will show "0012" 

Of cause this only works for positive integers of up to 4 digits. A slightly more complex solution, will handle positive integers:

'0'.repeat( Math.max(4 - num.toString().length, 0)) + num 

Create a string by repeat adding zeros, as long as the number of digits (length of string) is less than 4 Add the number, that is then converted to a string also.

Edit: from now on you should probably use this function:

String(num).padStart(4,'0') 

It still doesn't handle negative numbers...

Answer 2

Since ES2017 padding to a minimum length can be done simply with String.prototype.padStart and String.prototype.padEnd:

let number = 3 let string = number.toString().padStart(3, '0') console.log(string) // "003" 

Or if only the whole part of a float should be a fixed length:

let number = 3.141 let array = number.toString().split('.') array[0] = array[0].padStart(3, '0') let string = array.join('.') console.log(string) // "003.141" 

Neither of these simple uses handle sign, only showing a fraction part when number is not an integer, or other scenarios - so here is a simple example formatting function without options:

function format (number) {     let [ integer, fraction = '' ] = number.toString().split('.')     let sign = ''     if (integer.startsWith('-')) {         integer = integer.slice(1)         sign = '-'     }     integer = integer.padStart(3, '0')     if (fraction) {         fraction = '.' + fraction.padEnd(6, '0')     }     let string = sign + integer + fraction     return string }  console.log(format(3)) // "003" console.log(format(-3)) // "-003" console.log(format(4096)) // "4096" console.log(format(-3.141)) // "-003.141000" 

Although notably this will not handle things that are not numbers, or numbers that toString into scientific notation.