The tilde (~) is the bitwise NOT operator. ~x is roughly the same as -(x+1).
The double tilde (~~) is simply two single tilde operators, and so performs a bitwise complement twice. The side-effect is that the value gets converted to an integer number. Hence, ~~ is often used as a faster alternative to Math.floor.
console.log(Math.floor(42.01))
// 42
console.log(~~42.01)
// 42