Large numeric literals are difficult for the human eye to parse quickly, especially when there are lots of repeating digits:

1000000000000
   1019436871.42

To improve readability, a new JavaScript language feature enables underscores as separators in numeric literals. So, the above can now be rewritten to group the digits per thousand, for example:

1_000_000_000_000
    1_019_436_871.42

Now it’s easier to tell that the first number is a trillion, and the second number is in the order of 1 billion.

Numeric separators help improve readability for all kinds of numeric literals:

// A decimal integer literal with its digits grouped per thousand:
1_000_000_000_000
// A decimal literal with its digits grouped per thousand:
1_000_000.220_720
// A binary integer literal with its bits grouped per octet:
0b01010110_00111000
// A binary integer literal with its bits grouped per nibble:
0b0101_0110_0011_1000
// A hexadecimal integer literal with its digits grouped by byte:
0x40_76_38_6A_73
// A BigInt literal with its digits grouped per thousand:
4_642_473_943_484_686_707n

They even work for octal integer literals (although I can’t think of an example where separators provide value for such literals):

// A numeric separator in an octal integer literal: 🤷‍♀️
0o123_456

Note that JavaScript also has a legacy syntax for octal literals without the explicit 0o prefix. For example, 017 === 0o17. This syntax is not supported in strict mode or within modules, and it should not be used in modern code. Accordingly, numeric separators are not supported for these literals. Use 0o17-style literals instead.

Support for numeric separators #

• Chrome: supported since version 75
• Firefox: supported since version 70
• Safari: supported since version 13
• Node.js: supported since version 12.5.0
• Babel: supported