JavaScript Hoisting

JavaScript hoisting is basically that declarations are moved to the top of your code.

Actually your function and variable declarations are added to memory during the compile phase.
And they stay exactly where you typed them in your code.

JavaScript hoists all declarations

JavaScript hoists all declarations. Initializations are not hoisted.

Declare and initialize a variable, e.g. var x = 8;, only the var x; part (the declaration) is hoisted.
The x = 8; (the initialization) is not hoisted.

A variable that is declared but not initialized is automatically set as undefined.
Take a look at the examples to understand this.

Hoisting Variables Examples

// This is how you would expect to write the code
var x = 8; // Initialize and assign 8 to x
console.log('X has the value: '+ x);  // The printed text is: "X has the value: 8"
// Here y is assigned before it is declared
y = 8; // Assign 8 to y
console.log('Y has the value: '+ y);  // The printed text is: "Y has the value: 8"
var y; // Declare y

Learn more about JavaScript Variables

Hoisting Variables Undefined Examples

// Declare and assign the variable at the end
// Only the declaration is hoisted
console.log('Z has the value: '+ z);  // The printed text is: "Z has the value: undefined"
var z = 8; // Declare and assign 8 to z

Hoisting Functions Examples

// This is how you would expect to write the code
function printName(name) {
  console.log("My name is " + name);
printName("Donald");  // The printed text is: "My name is Donald"
// Here the function is called before it is declared
printCity("London");  // The printed text is: "My favorite city is London"
function printCity(name) {
  console.log("My favorite city is " + name);

Learn more about JavaScript Functions

Let and Const Statement

Variables and constants declared with let or const are not hoisted!

Learn more about JavaScript Let Learn more about JavaScript Const

Hoisting Best Practices

Hoisting is not well known to many developers.

It is considered a best practice to declare variables at the top of their respective scopes. Then you would limit undesirable effects.
Always initialize variables when they are declared. This will provide better code and avoid undefined variables.

JavaScript strict mode gives an error if variables is used before they are declared.

Learn more about JavaScript Scope

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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.

The benefits of using JavaScript are:

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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