In JavaScript, let
and var
are both used to declare variables. However, they have some key differences that you should be aware of.
Scope
The main difference between let
and var
is that let
has block-level scope, while var
has function-level scope. This means that a let
variable is only available within the block of code in which it is declared, while a var
variable is available throughout the entire function in which it is declared.
For example:
if (true) {
let x = 5;
}
console.log(x); // ReferenceError: x is not defined
if (true) {
var y = 5;
}
console.log(y); // 5
In the example above, the let
variable x
is only available within the block of code inside the if
statement, and an error is thrown when we try to access it outside the block. On the other hand, the var
variable y
is available throughout the entire function, and we are able to access it outside the block.
Temporal Dead Zone
Another difference between let
and var
is that let
variables are subject to the “temporal dead zone” (TDZ). This means that a let
variable is not accessible until it has been fully declared.
For Example:
console.log(x); // ReferenceError: x is not defined
let x = 5;
In the example above, an error is thrown when we try to access the let
variable x
before it has been fully declared. On the other hand, a var
variable is accessible before it has been fully declared, as shown in the example below:
console.log(y); // undefined
var y = 5;
In the example above, the var
variable y
is accessible before it has been fully declared, and its value is undefined
.
Re-declaration:
A let
variable cannot be re-declared within the same block of code, but a var
variable can be re-declared within the same function.
For example:
let x = 5;
let x = 10; // SyntaxError: Identifier 'x' has already been declared
var y = 5;
var y = 10; // no error
In the example above, an error is thrown when we try to re-declare the let
variable x
, but no error is thrown when we re-declare the var
variable y
.
In general, it is recommended to use let
over var
whenever possible, as let
has more intuitive behavior and can help prevent common errors. However, var
may still be used in certain cases, such as when you need to declare a variable with function-level scope.