JavaScript Let

Let is a keyword that is used to declare a variable.

You should use let instead of var when declaring a variable.

One of the most important things is that you should try to keep things simple!


Var, const and let

When Should You Use var, const Or let?

It is important to understand Global-Scope, Function-Scope and Block-Scope to know when you may use the different statements.

Global Scope

Variables declared Globally (outside any block or function) have Global Scope.

Be careful with using Global Scope.

var car = "Byton";
 
// code CAN use variable car
 
function carFunction() {
     // code CAN also use variable car 
}

Function Scope

Variables declared inside a function have Function Scope.

// code can NOT use variable car
 
function carFunction() {
     var car = "Byton";
     // code CAN use variable car
}
 
// code can NOT use variable car

Block Scope

Variables declared inside a block {} have Block Scope.

{ 
     var car = "Byton"; 
}
// code CAN use variable car

 

 


Learn more about JavaScript Scope

Let variables are block-scoped

let variables are block-scoped! Specific scope => less mistakes.

To keep things simple - Use const to identify a constant. Use let to define a variable.

Take a look at the example to understand this.

JS JavaScript - Let variables are block-scoped

// Variable car in "global" block-scope
let car = "Tesla";
 
if (true) {
  // Variable car in "local" block-scope
  // This will not affect the "global" declaration of car
  let car = "Polestar";
}
 
console.log(car); // "Tesla"

Learn more about JavaScript Variables Learn more about JavaScript Var

// Variable car in "global" block-scope
var car = "Tesla";
 
{
    // Variable car in "local" block-scope
    // This will not affect the "global" declaration of car 
    let car = "Polestar";
}
 
console.log(car); // "Tesla"

Keep it simple and use let instead of var.


// Declaring a variable using var
var x = 1;
for (var x = 0; x < 10; x++) {
    // Some statements
    // Incrementing x affects x that is declared outside this scope
}
// Here x is 10

Learn more about JavaScript Assignment Learn more about JavaScript Comments

// Declaring a variable using let
let x = 1;
for (let x = 0; x < 10; x++) {
    // Some statements
    // Incrementing x does not affect x that is declared outside this scope
}
// Here x is 1

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JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

3.      JavaScript to program the behavior of web pages.

JavaScript is a scripting or programming language that allows you to implement complex things on web pages. Every time a web page does more than just sit there and display static information for you to look at, displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. − you can bet that JavaScript is probably involved.


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1.      You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

2.      Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

3.      Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

4.      Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.


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