This feature enables developers to make their numeric literals more readable by creating a visual separation between groups of digits. Large numeric literals are difficult for the human eye to parse quickly, especially when there are long digit repetitions.

1000000000   // Is this a billion? a hundred millions? Ten millions?
101475938.38 // what scale is this? what power of 10?

Using underscores _as separators helps improve readability for numeric literals.

1_000_000_000           // Ah, so a billion
101_475_938.38          // And this is hundreds of millions

This proposal is currently on stage 4 , if you wanna more details you can find them here : l[github.com/tc39/proposal-numeric-separator]

enjoy !!