Array.prototype.multiply = function(){
	const le = this.length;
	for(let i=0; i < le; i++){
		this[i+le] = this[i]*this[i];
		console.log(this)
	}
};

const a = [1, 2, 3, 4, 5];
a.multiply();
console.log(a); //[1, 2, 3, 4, 5, 1, 4, 9, 16, 25]