A variable declaration is simply telling the computer that a variable exists and what value it should start with. Similar to other programming languages JavaScript also allows for the declaration of variables.
There are three keywords in JavaScript that can be used to declare variables: let, var, and const. Each keyword has different rules and implications for how the variables they create can be used.
- let: The let keyword declares a block-scoped local variable, optionally initializing it to a value.
Block-scoped means that the variable is only available within the block it was declared in, which is usually denoted by curly braces {}.
- var: The var keyword declares a function-scoped or global variable, optionally initializing it to a value.
Function-scoped means that the variable is only available within the function it was declared in. Global variables are available throughout your entire code.
- const: The const keyword declares a block-scoped, immutable constant variable, i.e. a variable that can’t be reassigned.
Constants are also called “immutable variables”, but that’s a bit of a misnomer since they are actually variables – just ones that can’t be reassigned.
What is difference between var, let and const?
The var keyword is the oldest way of declaring variables in JavaScript and is supported by all browsers. The let and const keywords are newer additions to the language and are not supported by older browsers.
If you need to support older browsers, you can use var instead of let or const. If you don’t need to support older browsers, you can use let or const. If you want your variable to be immutable, use const.
Here are some examples:
1 2 3 4 5 6 7 | var x = 1; let y = 2; const z = 3; x = 4; //OK y = 5; //OK z = 6; //Error |
As you can see, var and let variables can be reassigned, but const variables can not.
Another difference between var and let/const is that var variables are function-scoped, while let and const variables are block-scoped.
This means that var variables are only available within the function they were declared in. For example:
1 2 3 4 5 6 | function foo() { var x = 1; } foo(); console.log(x); // ReferenceError: x is not defined |
On the other hand, let and const variables are only available within the block they were declared in. For example:
1 2 3 4 5 6 7 8 | function foo() { let y = 2; const z = 3; } foo(); console.log(y); // ReferenceError: y is not defined console.log(z); // ReferenceError: z is not defined |
So, to sum up, the main differences between var, let and const are:
- var is function-scoped while let and const are block-scoped.
- var variables can be reassigned while let and const variables can not.
- var variables are declared using the var keyword while let and const variables are declared using the let and const keywords respectively.
- const variables are immutable while let and var variables are not.