Numeric separators allow adding underscores to numeric literals to make them more readable.
Examples
Looks like you can add numeric separators anywhere in a string literal except as the first character, last character, right after a negative, or next to a decimal point.
Please excuse any wacky syntax highlighting, they don't always do the best with new features.
let hard_to_read = 1000000;
let oh_now_i_see = 1_000_000;
let oh_now_i_see_with_decimals = 1_000_000.00_00;
let nope1 = _100; // Error: _100 is not defined
let nope2 = -_1; // Error: _1 is not defined
let nope3 = 100_; // Error: Numeric separators are not allowed here
let nope4 = 100_.0; // Error: Numeric separators are not allowed here