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JavaScript Primer Edit Page


Many new features were introduced to JavaScript with the release of newer specifications like ECMAScript 2015, also known as ECMAScript 6 or ES6. While the Guides assume you have a working knowledge of JavaScript, not every feature of the JavaScript language may be familiar to the developer.

In this guide we will be covering some JavaScript features, and how they are used in Ember applications.

Variable declarations

A variable declaration, also called binding, is when you assign a value to a variable name. An example of declaring a variable containing the number 42 is like so:

var myNumber = 42;

JavaScript initially had two ways to declare variables, globally and var. With the release of ES2015, const and let were introduced. We will go through the different ways to declare a variable, also called bindings because they bind a value to a variable name, and why modern JavaScript tends to prefer const and let.

var

Variable declarations using var exist in the entire body of the function where they are declared. This is called function-scoping, the existence of the var is scoped to the function. If you try to access a var outside of the function it is declared, you will get an error that the variable is not defined.

For our example, we will declare a var named name. We will try to access it both inside the function and outside, and see the results we get:

console.log(name); // ReferenceError: name is not defined

function myFunction() {
  var name = "Tomster";

  console.log(name); // "Tomster"
}

This also means that if you have an if or a for in your code and declare a var inside them, you can still access the variable outside of those blocks:

console.log(name); // undefined

if (true) {
  var name = "Tomster";

  console.log(name); // "Tomster"
}

In the previous example, we can see that the first console.log(name) prints out undefined instead of the value. That is because of a feature of JavaScript called hoisting. Any variable declaration is moved by the programming language to the top of the scope it belongs to. As we saw at the beginning, var is scoped to the function, so the previous example is the same as:

var name;
console.log(name); // undefined

if (true) {
  name = "Tomster";

  console.log(name); // "Tomster"
}

const and let

There are two major differences between var and both const and let. const and let are both block-level declarations, and they are not hoisted.

Because of this they are not accessible outside of the given block scope (meaning in a function or in {}) they are declared in. You also cannot access them before they are declared, or you will get a ReferenceError.

console.log(name) // ReferenceError: name is not defined

if (person) {
  console.log(name) // ReferenceError: name is not defined

  let name = 'Gob Bluth'; // "Gob Bluth"
} else {
  console.log(name) // ReferenceError: name is not defined
}

const declarations have an additional restriction, they are constant references, they always refer to the same thing. To use a const declaration you have to specify the value it refers, and you cannot change what the declaration refers to:

const firstName; // Uncaught SyntaxError: Missing initializer in const declaration
const firstName = 'Gob';
firstName = 'George Michael'; // Uncaught SyntaxError: Identifier 'firstName' has already been declared

Note that const does not mean that the value it refers to cannot change. If you have an array or an object, you can change their properties:

const myArray = [];
const myObject = { name: "Tom Dale" };

myArray.push(1);
myObject.name = "Leah Silber";

console.log(myArray); // [1]
console.log(myObject); // {name: "Leah Silber"}

for loops

Something that might be confusing is the behaviour of let in for loops.

As we saw before, let declarations are scoped to the block they belong to. In for loops, any variable declared in the for syntax belongs to the loop's block.

Let's look at some code to see what this looks like. If you use var, this happens:

for (var i = 0; i < 3; i++) {
  console.log(i) // 0, 1, 2
}

console.log(i) // 3

But if you use let, this happens instead:

for (let i = 0; i < 3; i++) {
  console.log(i) // 0, 1, 2
}

console.log(i) // ReferenceError: i is not defined

Using let will avoid accidentally leaking and changing the i variable from outside of the for block.

Promises

A Promise is an object that may produce a value some time in the future: either a resolved value, or a reason that it’s not resolved (e.g., a network error occurred). A Promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promises were introduced in ES6 JavaScript.

Why are Promises needed in Ember? JavaScript is single threaded, and some things like querying data from your backend server take time, thus blocking the thread. It is efficient to not block the thread while these computations or data fetches occur - Promises to the rescue! They provide a solution by returning a proxy object for a value not necessarily known when the Promise is created. While the Promise code is running, the rest of the code moves on.

For example, we will declare a basic Promise named myPromiseObject.

let myPromiseObject = new Promise(function(resolve, reject) {
  // on success
  resolve(value);

  // on failure
  reject(reason);
});

Promises come equipped with some methods, out of which then() and catch() are most commonly used. You can dive into details by checking out the reference links. .then() always returns a new Promise, so it’s possible to chain Promises with precise control over how and where errors are handled.

We will use myPromiseObject declared above to show you how then() is used:

myPromiseObject.then(function(value) {
  // on fulfillment
}, function(reason) {
  // on rejection
});

Let's look at some code to see how they are used in Ember:

store.findRecord('person', 1).then(function(person) {
  // Do something with person when promise is resolved.
  person.set('name', 'Tom Dale');
});

In the above snippet, store.findRecord('person', 1) can make a network request if the data is not already present in the store. It returns a Promise object which can resolve almost instantly if the data is present in store, or it can take some time to resolve if the data is being fetched by a network request.

Now we can come to part where these promises are chained:

store.findRecord('person', 1).then(function(person) {

  return person.get('post'); //get all the posts linked with person.

}).then(function(posts){

  myFirstPost = posts.get('firstObject'); //get the first post from collection.
  return myFirstPost.get('comment'); //get all the comments linked with myFirstPost.

}).then(function(comments){

  // do something with comments
  return store.findRecord('book', 1); //query for another record

}).catch(function(err){

  //handle errors

})

In the above code snippet, we assume that a person has many posts, and a post has many comments. So, person.get('post') will return a Promise object. We chain the response with then() so that when it's resolved, we get the first object from the resolved collection. Then, we get comments from it with myFirstPost.get('comment') which will again return a promise object, thus continuing the chain.

Resources

For further reference you can consult Developer Network articles: