Simple implementation of a Stack data structure in js

Stack.js

var Stack = function(capacity){
	this.capacity = capacity || Infinity
	this.storage = {}
	this.size = 0
}

Stack.prototype.push = function(val){
	if(this.size < this.capacity){
		this.storage[this.size] = value
		return this.size
	}
	
	return "Max capacity exceeded. Remove an element before adding a new one"
}

Stack.prototype.pop = function(){
	let value = this.storage[--this.size]
	delete this.storage[this.size]
	if(this.size < 0){
		this.size = 0
	}
	
	return value
	
}

Stack.prototype.size = function(){
	return this.size
}

Stack.prototype.peek = function(){
	return this.storage[this.size - 1]
}