If you are writing javascript code for Node.js, Babel, Typescript or if you can target up-to-date browsers, chances are that your environment supports the let and const keywords.

If this is the case, you should stop using var and use let and const exclusively. Here’s why:

var uses something called ‘hoisting’, which can lead to unexpected results.

uses something called ‘hoisting’, which can lead to unexpected results. let and const are both block-scoped. Which means you can declare them in for loop or if statement, and they will only be valid for that block. This helps with spotting bugs and makes your code more robust.

and are both block-scoped. Which means you can declare them in loop or statement, and they will only be valid for that block. This helps with spotting bugs and makes your code more robust. const prevents variable re-assignment.

Unless you are targetting older environments, there’s no need anymore to use var .

But the reason I’m writing this post is because I often see let used where const is more appropriate.

When to use const

const stands for “constant”. In a lot of sources, I see that interpreted as something that should never change, maybe even something that should be declared as uppercase:

/* Hear ye, Hear ye! I declare a constant! */ const MY_MAGIC_VALUE = 5 ;

However, const really should be your default for most cases. Take the following examples:

const user = {}; user . firstName = ' Evert ' ; user . lastName = ' Pot ' ;

When you create an object with const , you can still change it’s contents. const only prevents re-assigning, it doesn’t make the entire object immutable.

const users = []; users . push ( user );

Same for the array. Even though I’m adding something to the array, the identity of the array remains the same. const works.

for ( const item of users ) { console . log ( user ); }

In the case for the for loop. Every iteration of the loop is a new ‘block scope’, so I am in fact able to re-create a new constant for every iteration.

It’s useful to use const instead of let , because it prevents you from accidentally overwriting variables. So a good rule of thumb is:

Stop using var . Use const by default, everywhere. Use let if you must.

When to use let

There certainly are cases where let is needed. For example:

let x = 5 ; x ++ ;

The above example does not work const , because we’re really redefining x . Under the hood javascript runs x = x + 1 , which sets a new value (6) to x .

If this is confusing, the following example might help:

let x = 5 ; let y = x ; x ++ ; console . log ( x , y ); // output 6, 5

In the above example, we changed x , but y remained 5. This is because x++ re-assigns x to a new value, but the old value didn’t change.

Contrast this with changing an object value:

const x = [ 1 , 2 ]; const y = x ; x . push ( 3 );

After this script, both x and y contain [1, 2, 3] . Both variables refer to the same value. We changed the value, but did not re-define the variable.

In contrast, this will throw an error:

const x = [ 1 , 2 , 3 ]; const y = x ; const x = x . slice ( 1 );

Here we re-define x , and javascript doesn’t allow this. If we used let instead, only x would have changed but y would not.

But what if I want to declare a magic constant-like value in my source

Have you considered SHOUTING?