This is an article with the notes that I summarized after learning「Introduction To JavaScript」in Codecademy.

Iterators

Iterators are methods that are called on arrays and complete tasks of looping through the arrays.

.forEach() method

let fruits = ['mango', 'apple', 'pineapple', 'banana'];

// Iterate over fruits 
fruits.forEach(fruit => console.log("I want to eat a " + fruit));

// expected outputs:
// I want to eat a mango
// I want to eat a apple
// I want to eat a pineapple
// I want to eat a banana

In this method .forEach() iterates through the whole list fruits, and each time it iterates an item, it will log the sentence I want to eat a fruit into the console.

.map() method

let words = ['Hey', 'elephant', 'listen', 'list', 'old', 
             'What', 'octopus', 'rabbit', 'lie', 'deep'];

// iterates through each word
// and changes the word to the new returned word
let secretMessage = words.map(word => word[0]);

console.log(secretMessage.join('');
// HelloWorld

This method is similar to the former method, it still can iterates through the whole list. The difference is that each time it iterates through a word, the new returned word will replace the original word. In this case the returned word is word[0], which is the first letter of each word. So after the complete iteration, the list words will become ['H', 'e', 'l', 'l, 'o', 'W', 'o', 'r', 'l', 'd']. And with the method .join(''), all the items in the list will be joined into a single string HelloWorld.

.filter() method

let favoriteFood = ['tempeh', 'strawberry', 'orange', 'bread', 'cereal'];

let longFavoriteFood = favoriteFood.filter(food => food.length > 5);
// filters out all words that have length smaller than or equal to 5

console.log(longFavoriteFood);
// ['tempeh', 'strawberry', 'orange', 'cereal']

This method is used to filter an array. In this example, each time it loops through an item, it will look at the length property of that item. If its length is smaller than or equal to 5, this item will be filtered out. In this end, only the items that have length bigger than 5 will be returned.