JavaScript — Use Numeric Separators for Better Readability

Large numbers are hard to read. For example, what about “2014010167“? Is it 20 million, 200 million, or 2 billion? Yeah, we can’t read that number either and need to concentrate. What about “2014010_167”? Much better!

JavaScript shipped a new feature called “numeric separator” to improve readability on numbers. The underscore _ was introduced in ES2021 as a separator in numbers. You can use it for decimal, binary, hexadecimal, and BigInt numbers.

This tutorial walks you through the usage of numeric separators in JavaScript and how to use it.

Node.js Series Overview

  1. String Replace All Appearances
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  22. Get the Part Before Last Occurrence in a String in JavaScript or Node.js
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  24. How to Count Words in a File
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  26. Append Characters or Words to a String in JavaScript or Node.js (Coming soon)
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  31. Lowercase the First Character of a String in JavaScript or Node.js (Coming soon)
  32. Uppercase the First Character of a String in JavaScript or Node.js (Coming soon)
  33. Prepend Characters or Words to a String in JavaScript or Node.js (Coming soon)

Position of the Numeric Separator

A numeric separator can appear anywhere inside a number. Here’s a list of examples using numeric separators:

// separators in decimal numbers
2_014_010_167  
2_014.01

// separators in binary numbers
0b1010_0001_1001

// separators in octal numbers
0o2_2_3_4

// separators in hex numbers
0xA0_B0_C0

// separators in BigInts
2_014_010_167_000_000_000_000n  

It’s not allowed to put a numeric separator at the end of a number. This is invalid:

199_95_ // SyntaxError  

It’s also not allowed to put more than one separator in a row. This is also invalid:

199__95 // SyntaxError  

Another invalid position of a numeric separator is after a leading zero. This throws a syntax errors as well:

0_199; // SyntaxError  

In case you’re starting a number with _ it’s treated as a variable. This is not handled as a number because JavaScript variables can start with an underscore:

const invalid = _199_95  
// requires variable "_199_95" to exist or
// it fails with a ReferenceError

Remember: numeric separators are available since ES201 (or ES12). You may already be able to use it when you’re using a modern code base or transpiling to modern platforms.

Enjoy numeric separators!

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