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How to Output Numbers With Leading Zeros in JavaScript?

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Sometimes, we want to output numbers with leading zeroes in our JavaScript programs.

In this article, we’ll look at how to create numbers with leading zeroes with JavaScript.

String.prototype.padStart

The JavaScript string’s padStart method lets us add characters we want to a string until it reaches a given length.

For instance, we can write:

const padded = (123).toString().padStart(5, '0')
console.log(padded)

We call toString on the number to convert it to a string.

Then we call padStart with 5 to pad the string to length 5.

And we pass in '0' to pad the number with leading 0’s.

Therefore, padded is '00123' .

Write Our Own Code

We can also write our own code to pad a number string with leading zeroes.

For instance, we can write:

const zeroPad = (num, places) => {
  const numZeroes = places - num.toString().length + 1;
  if (numZeroes > 0) {
    return Array(+numZeroes).join("0") + num;
  }
  return num
}

console.log(zeroPad(5, 2))
console.log(zeroPad(5, 4))
console.log(zeroPad(5, 6))

We create the zeroPad function that takes the num and places parameters.

num has the number we want to pad with leading zeroes.

places is the number of decimal places that we want to pad to.

In the function, we compute the numZeroes variable, which is the number of leading zeroes we want to add to make the string the length set by places .

We compute that by substracting places with num.toString().length plus 1.

This subtracts the final number of places with the length of the number string plus 1.

We add 1 so that we create the correct sized array since we want to add zeroes in between the array entries with join .

Then if numZeroes is bigger than 0, we return a string with leading zeroes by creating an array, then calling join with 0 to create the zero string, and then concatenating that with the num value.

Otherwise, we return num itself.

So the console log should log:

05
0005
000005

Conclusion

We can pad our JavaScript nuinmber strings with leading zeroes with the padStart method or write our own code to do so.

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