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Variable declaration: var, const, let

In JavaScript, there are three keywords available to declare a variable, and each has its differences. Those are var, let and const.

Short explanation

Variables declared with const keyword can’t be reassigned, while let and var can.

I recommend always declaring your variables with const by default, but with let if it is a variable that you need to mutate or reassign later.

Sample code

**const** person **=** "Nick";
person **=** "John" *// Will raise an error, person can't be reassigned*

**let** person **=** "Nick";
person **=** "John";
console.log(person) *// "John", reassignment is allowed with let*

Detailed explanation

The scope of a variable roughly means “where is this variable available in the code”.

var

var declared variables are function scoped, meaning that when a variable is created in a function, everything in that function can access that variable. Besides, a function scoped variable created in a function can’t be accessed outside this function.

I recommend you to picture it as if an X scoped variable meant that this variable was a property of X.

**function** myFunction() {
  **var** myVar **=** "Nick";
  console.log(myVar); *// "Nick" - myVar is accessible inside the function*
}
console.log(myVar); *// Throws a ReferenceError, myVar is not accessible outside the function.*

Still focusing on the variable scope, here is a more subtle example:

**function** myFunction() {
  **var** myVar **=** "Nick";
  **if** (**true**) {
    **var** myVar **=** "John";
    console.log(myVar); *// "John"// actually, myVar being function scoped, we just erased the previous myVar value "Nick" for "John"*
    }
  console.log(myVar); *// "John" - see how the instructions in the if block affected this value*
  }
console.log(myVar); *// Throws a ReferenceError, myVar is not accessible outside the function.*

Besides, var declared variables are moved to the top of the scope at execution. This is what we call var hoisting.