JavaScript Behavioral Design Patterns Iterator
Example
An iterator pattern provides a simple method for selecting, sequentially, the next item in a collection.
Fixed Collection
class BeverageForPizza {
constructor(preferenceRank) {
this.beverageList = beverageList;
this.pointer = 0;
}
next() {
return this.beverageList[this.pointer++];
}
var withPepperoni = new BeverageForPizza(["Cola", "Water", "Beer"]);
withPepperoni.next(); //Cola
withPepperoni.next(); //Water
withPepperoni.next(); //Beer
In ECMAScript 2015 iterators are a built-in as a method that returns done and value. done is true when the iterator is at the end of the collection
function preferredBeverage(beverage){
if( beverage == "Beer" ){
return true;
} else {
return false;
}
}
var withPepperoni = new BeverageForPizza(["Cola", "Water", "Beer", "Orange Juice"]);
for( var bevToOrder of withPepperoni ){
if( preferredBeverage( bevToOrder ) {
bevToOrder.done; //false, because "Beer" isn't the final collection item
return bevToOrder; //"Beer"
}
}
As a Generator
class FibonacciIterator {
constructor() {
this.previous = 1;
this.beforePrevious = 1;
}
next() {
var current = this.previous + this.beforePrevious;
this.beforePrevious = this.previous;
this.previous = current;
return current;
}
}
var fib = new FibonacciIterator();
fib.next(); //2
fib.next(); //3
fib.next(); //5
In ECMAScript 2015
function* FibonacciGenerator() { //asterisk informs javascript of generator
var previous = 1;
var beforePrevious = 1;
while(true) {
var current = previous + beforePrevious;
beforePrevious = previous;
previous = current;
yield current; //This is like return but
//keeps the current state of the function
// i.e it remembers its place between calls
}
}
var fib = FibonacciGenerator();
fib.next().value; //2
fib.next().value; //3
fib.next().value; //5
fib.next().done; //false
