JS: 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