JavaScript Primer

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.

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"
}

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 can also not 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"}

Something that might be confusing is the behavior 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.

For further reference you can consult Developer Network articles:

• var
• const
• let.