var, let & const are the three javascript keywords which are used to define the properties of a variable. Based on the restrictiveness on variable, we choose one of these keywords. In this article I will discuss the difference between var, let and const keywords in JavaScript.
Difference between var, let & const
var
1. Variables declared by var can be redeclared without any issue. Check this code –
var a = 1; var a = 2;
The above code is completely valid.
2. var can be declared and initialized in separate steps. Check the code –
var a; a = 1 // Valid ✔️
3. Values can be reassigned. Check this code –
var a = 1; a = 2; // Valid ✔️
4. They are not bound with scopes. That is, a var variable value will be same inside and outside the scope. Check this code –
var a = 1
console.log(a) // Output: 1
{
console.log(a) // Output: 1
a = 5
console.log(a) // Output: 5
}
console.log(a) // Output: 5
a = 6
{
console.log(a) // Output: 6
}
In the above code we saw that value of a remained same everywhere if not reassigned to something else.
5. Need not be declared before use. Check the code –
a = 1; var a; // Valid ✔️
The above code is valid.
let & const
let & const behave nearly the same except that you can reassign a let variable but not const.
1. You can’t redeclare a let or const value. Check this code –
let a = 5; let a = 6; // INVALID ❌ const b = 5; const b = 6; // INVALID ❌ const a = 5; // INVALID ❌ let b = 5 // INVALID ❌
In the above code you can see that once a variable is declared using either let or const, you can’t redeclare it with in a scope.
2. You can declare a let variable and initialize it later but not const. const variables need to be initialized at the time of declaration. Check the code –
let a; a = 5 // Valid ✔️ const b; // Invalid ❌ b = 6 // Invalid ❌
3. let values can be reassigned but not const values. Check the code –
let a = 5 a = 6 // Valid ✔️ const b = 10 b = 11 // Invalid ❌
4. Both let and const values are bound to the scope. Check the code –
let a = 6
console.log(a) // Output: 6
{
console.log(a) // Output: 6
}
{
console.log(a) // Invalid ❌ because "a" is initialized
// in this scope so it will use that value
// But it is declared after console. var allows
// it but let doesn't
let a = 7 // Valid ✔️
console.log(a) // Output: 7
}
console.log(a) // Output: 6
let a = 8 // Invalid ❌ because "a" is already declared
// in 1st line
5. Both let and const need to be declared before use. Check the code –
a = 5 let a; // Invalid ❌
What should we use – var, let or const?
We should always use const. But if you need to reassign the value later in the code then you can use let. There is no general cases where var should be used.
