JavaScript: String.prototype.substring

str.substring(start, end)

Same as str.slice(start, end) but if any argument is negative, it is replaced with 0, and if any argument is greater than length, it's replaced by length.

[see String.prototype.slice]

const ss = "abcd";

console.log( ss.substring(1,3) ); // bc

console.log( ss.substring(0, ss.length) ); // abcd
console.log( ss.substring() ); // abcd
console.log( ss.substring(1) ); // bcd

console.log( ss.substring(-1) ); // abcd
console.log( ss.substring(-2) ); // abcd