Summary: in this tutorial, you’ll learn about JavaScript hoisting and how it works under the hood.
Introduction to the JavaScript hoisting
When the JavaScript engine executes the JavaScript code, it creates the global execution context. The global execution context has two phases: creation and execution.
During the creation phase, the JavaScript engine moves the variable and function declarations to the top of your code. This feature is known as hoisting in JavaScript.
Variable hoisting
Variable hoisting means the JavaScript engine moves the variable declarations to the top of the script.
For example, the following example declares the counter variable and initialize its value to 1:
console.log(counter); // undefined
var counter = 1;
In this example, we reference the counter variable before the declaration.
However, the first line of code doesn’t cause an error. The reason is that the JavaScript engine moves the variable declaration to the top of the script.
Technically, the code looks like the following in the execution phase:
var counter;
console.log(counter); // undefined
counter = 1;
During the creation phase of the global execution context, the JavaScript engine places the variable counter in the memory and initializes its value to undefined .
The let keyword
The following declares the variable counter with the let keyword:
console.log(counter);
let counter = 1;
The JavaScript issues the following error:
"ReferenceError: Cannot access 'counter' before initialization
The error message explains that the counter variable is already in the heap memory. However, it hasn’t been initialized.
Behind the scenes, the JavaScript engine hoists the variable declarations that use the let keyword. However, it doesn't initialize the let variables.
Notice that if you access a variable that doesn’t exist, the JavaScript will throw a different error:
console.log(alien);
let counter = 1;
Here is the error:
"ReferenceError: alien is not defined
Function hoisting
Like variables, the JavaScript engine also hoists the function declarations. This means that the JavaScript engine also moves the function declarations to the top of the script.
For example:
let x = 20,
y = 10;
let result = add(x, y);
console.log(result);
function add(a, b) {
return a + b;
}
In this example, we called the add() function before defining it. The above code is equivalent to the following:
function add(a, b){
return a + b;
}
let x = 20,
y = 10;
let result = add(x,y);
console.log(result);
During the creation phase of the execution context, the JavaScript engine places the add() function declaration in the heap memory.
To be precise, the JavaScript engine creates an object of the Function type and a function reference called add that refers to the function object.
Function expressions
The following example changes the add from a regular function to a function expression:
let x = 20,
y = 10;
let result = add(x,y);
console.log(result);
var add = function(x, y) {
return x + y;
}
If you execute the code, the following error will occur:
“TypeError: add is not a function
During the creation phase of the global execution context, the JavaScript engine creates the add variable in the memory and initializes its value to undefined.
When executing the following code, the add is undefined, hence, it isn't a function:
let result = add(x,y);
The add variable is assigned to an anonymous function only during the execution phase of the global execution context.
Arrow functions
The following example changes the add function expression to the arrow function:
let x = 20,
y = 10;
let result = add(x,y);
console.log(result);
var add = (x, y) => x + y;
The code also issues the same error as the function expression example because arrow functions are syntactic sugar for defining function expressions.
“TypeError: add is not a function
Similar to the functions expressions, the arrow functions aren’t hoisted.
Summary
JavaScript hoisting occurs during the creation phase of the execution context that moves the variable and function declarations to the top of the script.
The JavaScript engine hoists the variables declared using the let keyword, but it doesn't initialize them as the variables declared with the var keyword.
The JavaScript engine doesn’t hoist the function expressions and arrow functions.
Originally published at https://www.javascripttutorial.net.
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