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What is the difference between “let” and “var”?

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Dec 05.2022
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JavaScript

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@lestermDec 06.2022 — The main difference between var and let is that var is function scoped, while let is block scoped. This means that a variable declared with var is available for use within the whole function in which it is declared, while a variable declared with let is only available for use within the block in which it is declared. let also allows for more precise control over the visibility of variables, and it prevents variables from being redeclared or updated in the same scope, which can help avoid certain types of bugs. In general, it is recommended to use let instead of var for variable declarations in modern JavaScript code.

Here are two code examples that demonstrate the differences between var and let in terms of their scoping and redeclaration behavior.

In the first example, we declare a variable foo with var outside of a block, which makes it available for use within the whole function:

function example1() {
var foo = 'Hello';
if (true) {
// we can use 'foo' here because it is available in the whole function
console.log(foo); // prints 'Hello'
}
// we can also use 'foo' here because it is available in the whole function
console.log(foo); // prints 'Hello'
}


In the second example, we try to redeclare a let variable baz within the same block, which results in a SyntaxError:


function example2() {
let baz = 'Hello';
// we cannot redeclare 'baz' within the same block
let baz = 'Goodbye'; // throws a 'SyntaxError'
}


These examples show some of the key differences between var and let in terms of their scope and redeclaration behavior. In general, it is recommended to use let instead of var for variable declarations in modern JavaScript code.
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@JaySODec 05.2022 — In JavaScript, the let and var keywords are used to declare variables. The main difference between the two is that var is function-scoped, while let is block-scoped. This means that a variable declared with var is accessible within the entire function in which it is declared, while a variable declared with let is only accessible within the block in which it is declared.

For example:


function foo() {
if (true) {
var x = 1;
let y = 2;
}
console.log(x); // Output: 1
console.log(y); // Output: ReferenceError: y is not defined
}


Above, the x variable is declared with var inside the if block and is accessible outside the block, so the console.log statement after the if block can print its value. On the other hand, the y variable is declared with let inside the if block and is not accessible outside the block, so the console.log statement after the if block throws a ReferenceError because the y variable is not defined.

Another difference between var and let is that var variables are hoisted to the top of their scope, while let variables are not. This means that you can access a var variable before it is declared in the code, but you cannot access a let variable before it is declared. For example:


console.log(x); // Output: undefined
var x = 1;

console.log(y); // Output: ReferenceError: y is not defined
let y = 2;


In this code, the first console.log statement prints undefined for the x variable because the var keyword is hoisted to the top of the scope and the x variable is accessible before it is declared. On the other hand, the second console.log statement throws a ReferenceError because the let keyword is not hoisted and the y variable is not accessible before it is declared.

In general, let is a more modern and preferred way to declare variables in JavaScript because it is more flexible and safer than var. However, var is still widely used and supported in most JavaScript environments.

There is more info on JavaScript variables over at WebReference.
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