Had a mind-blown moment today when I learned that both javascript and ruby accept underscores in numbers. Underscores are used where commas or periods would be and are just ignored when parsing the number.
JavaScript
> console.log(1_000)
1000
undefined
> console.log(1_000 + 2_000)
3000
Ruby
> puts 1_000_000
1000000
=> nil
> puts 1_000 + 2_000
3000