As js is an interpreted language , functions and variables are defined on top during the compilation phase . This means that we can use functions and variables before they are declared in the code.

Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their containing scope during the compilation phase, before the code is executed. This allows you to use variables and functions before they are declared in the code. However, it's important to note that only the declarations are hoisted, not the initializations.

Variable Hoisting:

1console.log(x); // Output: undefined
2var x = 5;
3console.log(x); // Output: 5
4

In the example above, the variable x is hoisted to the top of its scope, but the initialization (var x = 5;) remains in its original position. So, when you try to log the value of x before its declaration, it outputs undefined.

Function Hoisting:

1foo(); // Output: "Hello, I am a function!"
2
3function foo() {
4  console.log("Hello, I am a function!");
5}

In this example, the entire function foo is hoisted to the top of its scope, including both the declaration and the function body. Therefore, you can call the function before its actual position in the code.

Hoisting with let and const:

Unlike var, let and const declarations are hoisted but not initialized. This is often referred to as the "temporal dead zone." If you try to access the value of a let or const variable before its declaration, it will result in a ReferenceError.

1console.log(y); // ReferenceError: Cannot access 'y' before initialization
2let y = 10;
3console.log(y); // Output: 10
4

1sayHello(); // TypeError: sayHello is not a function
2var sayHello = function() {
3  console.log("Hello!");
4};
5

In this case, the variable sayHello is hoisted, but its value is undefined at the point of the function call. This results in a TypeError since undefined is not a function.

Conclusion

Understanding hoisting is essential for writing predictable and error-free JavaScript code. It's recommended to declare variables and functions at the beginning of their scope to avoid confusion and unexpected behavior.