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

rated 0 times [ 124] [ 3] / answers: 1 / hits: 132789 / 15 Years ago, tue, july 14, 2009, 12:00:00

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.

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quinlanhenryf

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answered 15 Years ago rebekahalysah · Accepted Answer

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.