Array.prototype.insert

Array.insert.js

// My brain understands this better than .splice

/* var fruits = ["A","B","C"]; */

if ( typeof Array.insert !== "function" ) {
	Array.prototype.insert = function () {
		var args = Array.prototype.slice.call(arguments, 0);
		// Convert non arrays to array
		if ( args.length && !Array.isArray(args[0]) ) args[0] = [args[0]];
		// Set default position
		if ( args.length === 1 ) args.push(0);
		// Make negative pos count from the end
		if ( args[1] < 0 ) args[1] = args[1]+this.length+1;
		// Now stop invalid numbers
		if ( args[1] < 0 ) args[1] = 0;
		// Position out of bounds
		if ( args[1] > this.length ) args[1] = this.length;
		// Warn about usage
		if ( args.length !== 2 ) throw new Error("Array.insert takes one or two arguments [array, number]");
		// Apply insertion magic
		this.splice.apply(this, [args[1], 0].concat(args[0]));
		return this;
	};
}

/*
document.querySelector("div").innerHTML = fruits;
fruits.insert(["D", "E"], 0);
document.querySelector("p").innerHTML = fruits;
// <!-- <div></div><p></p> -->
*/