What are the differences between var, let and const in Javascript?

Javascript has been originally created with only one keyword to declare a variable, this reserved word is: var.

But since the 6th version (ECMAScript 2015) of the language specification, two new keywords appeared: let and const.

If I had to summarize the definition of these two new ways for declaring variables, I could say that:

• let is to declare a local variable
• const is to declare a constant variable

Be careful, a constant reference doesn't mean that the value behind the reference is immutable, only that the reference can't be changed.

But let's discover how these variables work differently in practice:

Store globally?

• var : yes✔️
• let : no ❌
• const : no ❌

When var is used to delcare a variable outside of a function, then this variable will be referenced in the global object of the script.

In the browser, this variable is called "window" and in a NodeJS environment it's called "global".

Limited to function scope?

• var : yes ✔️
• let : yes ✔️
• const : yes ✔️

The scope of a function is the only one to put all the variables on an equal footing and declaring one inside of a function won't have any effect on the outside variables.

Limited to block function?

• var : no ❌
• let : yes ✔️
• const : yes ✔️

To those who wonder what a block is, it simply is a group of instructions written between a pair of curly brackets "{...}" except when they delimiting a function body or a JSON object.

A set of instruction is ofter found after an if, else, for, while, etc. Remember that a variable declared using "var" inside a for will automatically be considered as global.

Excepted if the for is inside a function, obviously.

Can be reassigned?

• var : yes ✔️
• let : yes ✔️
• const : no ❌

Reassigning means either changing the value of the variable when we're using primitive types, or modifying the variable reference when using a complex type.

Do note that if const is used to reference an object, the content of this object can still be modified.

Can be redeclared?

• var : yes ✔️
• let : no ❌
• const : no ❌

Redeclaring a variable is impossible excepted if using var, but it's definitely not a good practice and I advise you not to do it.

Hoisting?

• var : yes ✔️
• let : no ❌
• const : no ❌

Hoisting is a more complex mechanism which happens during JS code interpretation, and if you're not familiar with this concept, I recommand you to read my dedicated blog post about it (french) !