Hoisting is a concept that enables us to extract values of variables and functions even before initializing/assigning value without getting errors and this happens during the 1st phase (memory creation phase) of the Execution Context.
Features of Hoisting:
- In JavaScript, Hoisting is the default behavior of moving all the declarations at the top of the scope before code execution. Basically, it gives us an advantage that no matter where functions and variables are declared, they are moved to the top of their scope regardless of whether their scope is global or local.
- It allows us to call functions before even writing them in our code.
Note: JavaScript only hoists declarations, not initializations.
JavaScript allocates memory for all variables and functions defined in the program before execution.
Sequence of variable declaration: The following is the sequence in which variable declaration and initialization occur.
Declaration –> Initialisation/Assignment –> Usage
Variable lifecycle:
let a; // Declaration
a = 100; // Assignment
console.log(a); // Usage
However, since JavaScript allows us to both declare and initialize our variables simultaneously, so we can declare and initialize at the same time.
let a = 100;
Note: Always remember that in the background the Javascript is first declaring the variable and then initializing them. It is also good to know that variable declarations are processed before any code is executed.
However, in javascript, undeclared variables do not exist until the code assigning them is executed. Therefore, assigning a value to an undeclared variable implicitly creates it as a global variable when the assignment is executed. This means that all undeclared variables are global variables.
Example:
function codeHoist() {
a = 10;
let b = 50;
}
codeHoist();
console.log(a);
console.log(b);
|
Output:
10
ReferenceError: b is not defined
Explanation: In the above code, we created a function called codeHoist() and in there we have a variable that we didn’t declare using let/var/const and a let variable b. The undeclared variable is assigned the global scope by javascript hence we are able to print it outside the function, but in case of the variable b the scope is confined and it is not available outside and we get a ReferenceError.
Note: There’s a difference between ReferenceError and undefined errors. An undefined error occurs when we have a variable that is either not defined or explicitly defined as type undefined. ReferenceError is thrown when trying to access a previously undeclared variable.
JavaScript var of ES5: When we talk about ES5, the variable that comes into our minds is var. Hoisting with var is somewhat different. When it is compared to let/const.
