See if the array contains the value, returns a boolean. Supports a detection between similar objects, but then requires the equalObjects function (https://gist.github.com/zykadelic/6020024).

array.includes.js

if(!Array.prototype.includes){
	Array.prototype.includes = function(value, detectSimilarObjects){
		if(typeof detectSimilarObjects === 'undefined' || typeof value !== 'object'){
			detectSimilarObjects = false;
		}
		if(detectSimilarObjects === true && typeof equalObjects !== 'function'){
			throw Error("Array.includes() does not recognize equalObjects(). Get it here: https://gist.github.com/zykadelic/6020024");
		}
		for(i = 0; i < this.length; i++){
			if(detectSimilarObjects ? equalObjects(this[i], value) : this[i] === value){
				return true;
			}
		}
		return false;
	}
}