A variable is just a holder of a value, like a box holding an item.
In JavaScript, you can create 3 types of variables: using const, let, and var statements. Each variable type has different behavior regarding the declaration, initialization, value access, and assignment steps.
This post will help you solidify your knowledge of JavaScript variables.
Let's get started.
First, let's see what a variable is and how it's identified.
In simple terms, a variable is a placeholder (or a box) for a value. A value in JavaScript can be either a primitive or an object.
The variable has a name, which stricter is called identifier. Examples of variable identifiers are myNumber, name, list, item.
The syntax of an identifier is pretty simple:
An identifier can contain letters, digits
0..9, and special symbols$,_. An identifier cannot start with a digit0..9.
Examples of valid identifiers are myNumber, my_number, list1, $item, _name, a, b, $, _.
Examples of invalid identifiers are 1number (incorrectly starts with the digit 1), my-number (the symbol - is not allow in an identifier), function (the identifier cannot be a keyword).
Finally,
JavaScript identifiers are case sensitive.
For example, the identifiers myNumber and mynumber are different, because the letter n has different cases in the identifiers (N vs n).
Having that, now let's see the 3 ways you can declare variables in JavaScript: const, let, and var statements.
Here's how you can declare a const variable with the identifier myConstVariable:
const myConstVariable = initialValue;
The variable myConstVariable is initialized right away with an initialValue, which can be any expression that evaluates to a value.
Examples of initial values are literals (numbers 1, strings 'My Initial Value'), other variables, function calls.
For example the statement:
Declares a const variable with the identifier hero initialized with the string literal 'Batman'.
What's also specific to const variables (contrary to let and var variables described below) is that you have to assign the initial value:
const hero; // SyntaxError: Missing initializer in const declaration
After the declaration and initialization, you can access the variable value just by using its name (aka identifier):
const hero = 'Batman';
console.log(hero); // logs 'Batman'
console.log(`Hello, ${hero}!`); // logs 'Hello, Batman!'
What distinguishes const variables from let and var is that you cannot assign a new value to a const variable:
const hero = 'Batman';
hero = 'Joker'; // TypeError: Assignment to constant variable
The code block and a function body create a scope for const variables. The concept of scope defines the limits where the variable is accessible.
In the example below hero variable is declared within the scope of the if conditional block. Thus, you can access hero only within that block, but not
outside:
if (true) {
// Code block scope
const hero = 'Batman';
console.log(hero); // logs 'Batman'
}
console.log(hero); // throws ReferenceError
Same way hero is declared inside the scope of the greetBatman() function:
function greetBatman() {
// Function scope
const hero = 'Batman';
console.log(`Hello, ${hero}!`); // logs 'Hello, Batman!'
}
console.log(`Hello, ${hero}!`); // throws ReferenceError
greetBatman();
On top of that, you can also access the const variable value only after the declaration statement, but not before:
console.log(hero); // throws ReferenceError
const hero = 'Batman';
From the usage perspective, you should use const variables as read-only that you don't plan to reassign.
Here's how you can declare a let variables with the identifiers myVariable1 and myVariable2:
let myVariable1 = initialValue;
// or
let myVariable2;
The variable myVariable1 is declared and initialized right away with an initialValue.
The variable myVariable2 is declared, however doesn't have an initial value. By default, JavaScript considers uninitialized variables having the special value undefined.
In the following code snippet:
let villain = 'Joker';
let name;
The let variable named villain is declared and initialized with the string literal 'Joker'. name variable also has been defined, but it wasn't initialized.
After the declaration and initialization, you can access the let variable value just by writing the identifier:
let villain = 'Joker';
console.log(villain); // logs 'Joker'
console.log(`Hello, ${villain}!`); // logs 'Hello, Joker!'
let name;
console.log(name); // logs undefined
name variable, while being declared, isn't initialized so it has an undefined value.
You can easily update the value of a let variable, a thing you cannot do with const variables - and that's the main difference between them.
let villain = 'Joker';
villain = 'Bane';
console.log(villain); // logs 'Bane'
The scope of the let variables is defined the same way as for const: by a code block or function body.
if (true) {
// Code block scope
let villain = 'Joker';
console.log(villain); // logs 'Joker'
}
console.log(villain); // throws ReferenceError
function greetJoker() {
// Function scope
let villain = 'Joker';
console.log(`Hello, ${villain}!`); // logs 'Hello, Joker!'
}
console.log(`Hello, ${villain}!`); // throws ReferenceError
greetJoker();
Same as with const, you cannot access a let variable before the declaration statement:
console.log(villain); // throws ReferenceError
let villain = 'Joker';
Here's how you can declare a var variable with the identifier myVariable1 or myVariable2:
var myVariable1 = initialValue;
// or
var myVariable2;
The variable myVariable1 is declared and initialized right away with an initialValue, which can be any kind of expression: literal, function call, etc.
The variable myVariable2 is declared, however doesn't have an initial value. By default, JavaScript considers uninitialized variables as having the special value undefined.
In the following code snippet:
var city = 'Gotham';
var name;
The var variable city is declared and initialized with the string literal 'Gotham'. name variable also has been defined, but it wasn't initialized.
After the declaration and optional initialization, you can access the var variable value just by using its name (or identifier):
var city = 'Gotham';
console.log(city); // logs 'Gotham'
console.log(`Welcome to ${city}!`); // logs 'Welcome to Gotham!'
var name;
console.log(name); // logs undefined
name variable, while being declared, it is not yet initialized so it has an undefined value.
Same as with let variables, you can easily reassign var variables:
var city = 'Gotham';
city = 'New York';
console.log(city); // logs 'New York'
In contrast to const and let, the scope of the var variables is defined only by the function body:
function welcomeTo() {
// Function scope
var city = 'Gotham';
console.log(`Welcome to ${city}!`); // logs 'Welcome to Gotham!'
}
console.log(`Welcome to ${city}!`); // throws ReferenceError
welcomeTo();
A code block doesn't create a scope for var variables:
if (true) {
// Code block scope
var city = 'Gotham';
console.log(city); // logs 'Gotham'
}
console.log(city); // logs 'Gotham'
Normally, you won't access a var variable before the declaration statement. But if you do, JavaScript won't throw a reference error, but rather evaluate the variable to undefined:
console.log(city); // logs undefined
var city = 'Gotham';
It happens because a var variables hoists up to the top of the scope.
There are 3 ways to declare variables in JavaScript: using const, let, or var statements.
Specific to const variable is that you have to initialize the variable with an initial value. Also, the const variable cannot be reassigned.
let, on the other side, can be declared with or without an initial value. Also let variable value can be updated.
var variables behave almost as let variables: can be initialized or not, as well can be reassigned. However, contrary to let and const, only the function body creates a scope for var variables.
Side challenge: what happens if you assign a value to a non-existing variable nonExisting = 'hello'?
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