Variables declared with var
These variables can be assigned new values later on without any problem.
Variables declared with let
Variable declaration with let is one of the two new methods of declaring variables that were introduced in ES6. A variable can be declared with let as :
These variables can also be assigned new values later on without any problem.
Variables declared with const
Variable declaration with const is another method of declaring variables that was introduced in ES6. A variable can be declared with const as :
These variables cannot be assigned new values later on because const declares a constant. A constant value is one that never changes so, if we try to reassign a constant variable, we will run into an error, i.e.
Note : const must be assigned a value at the time of its declaration, for example, this is not allowed
But this is not the case with let and var.
2. Scope differences
Variables declared with var in ES5 are function-scoped, but variables declared with let and const in ES6 are block-scoped. Now, what does that mean?
A variable declared with var anywhere in the function is accessible within the whole function. For example, if we declare a variable within an if statement inside a function, that variable is also accessible outside of that if block.
Whereas, if the variable is declared with let, it is only accessible inside the same block where it is declared. In this case, it is only accessible inside the if block and not accessible anywhere else outside that if block.
The same is the case with const.
Another difference is hoisting of variables declared with let and var. When using var, variables are set to 'undefined' during hoisting. For example, here the console statement prints undefined because the variable carModel is hoisted and is set to undefined.
In case of let and const, this does not happen. For example, here the console statement throws an error.
Does this mean that variables declared with let and const are not hoisted? No, the variables are hoisted but they are not set to undefined and are not available for use until they are declared. This happens because of something called the temporal-dead zone.
So, What to use for the variable Declaration?
If you are working with ES6, the best practice is to use let for variables that will change the value over time and const for variables that cannot be reassigned.
If you have any queries and suggestions, leave them in the comments below.😊
Top comments (1)
Thank you. This is Helpful.