JavaScript Hoisting refers to the behavior of variables and function declarations being moved to the top of their scope during the compilation phase, but assignments remain in place. 1. The function declaration is completely promoted, so it can be called before the declaration; 2. The variable declared using var is only promoted, the assignment remains in place, and accessing unassigned variables will return undefined; 3. Let and const are also promoted, but are not accessible before the declaration, otherwise a ReferenceError is thrown because it is in a temporary dead zone (TDZ); 4. Class declarations are also similar to let and const. Although they are promoted but not initialized, an error will be reported. Understanding the lift mechanism helps avoid code errors and improve debugging efficiency, the best practice is to always declare and then use variables and functions.
Hoisting in JavaScript is a behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase, before the code is executed. This means you can use functions or variables before they're declared in your code.
It might seem like JavaScript actually moves your code physically, but what really happens is that the declarations are processed first—during the creation phase of the execution context. Assignments (values ??given to variables) stay exactly where they are.
Let's break down how this works with different types of declarations.
Function Declarations Are Fully Hoisted
With function declarations, both the name and the body of the function are hoisted. That's why you can call a function before it appears in your code.
sayHello(); // Output: Hello!
function saysHello() {
console.log('Hello!');
} In this case, sayHello() works even though it's called before the actual declaration. The whole function is available from the start of its scope.
Just keep in mind that function expressions behave differently.
Variable Hoisting Only Moves the Declaration, Not the Assignment
Variables declared with var are partially hoisted—they're declared at the top of their scope, but any value assigned to them stays where it is in the code. If you try to access a variable before the assignment, you'll get undefined .
console.log(myName); // Output: undefined var myName = 'Alice'; console.log(myName); // Output: Alice
So during the initial phase:
-
myNameis declared and initialized withundefined - When the line
myName = 'Alice'runs, it updates the value
This can be confusing if you're not expecting it, especially when debugging issues related to variable usage.
Here's what you should remember about var hoisting:
- Declaration is hoisted
- Initialization stays in place
- Accessing it early returns
undefined, not an error
Let and Const Are Also Hoisted, But Not Initialized
Variables declared with let and const are hoisted too, but they aren't initialized. Trying to access them before the actual declaration results in a ReferenceError , because they enter something called the Temporal Dead Zone (TDZ).
console.log(age); // ReferenceError let age = 25;
Even though the declaration is hoisted, these variables remain inaccessible until the line where they're defined is reached.
Key points for let and const :
- They are hoisted
- They are not accessible before declaration
- You'll get a
ReferenceErrorif you try to use them too early
This behavior helps catch bugs caused by using variables before they're ready.
Class Declarations Are Also Hoisted, But Like Let and Const
Just like let and const , class declarations are hoisted but not initialized. So trying to create an instance of a class before its declaration will throw an error.
let person = new Person(); // ReferenceError
class Person {}Again, this prevents incorrect usage and encourages better code organization.
Hoisting isn't magic—it's just how JavaScript prepares your code behind the scenes. Understanding it helps avoid surprises when writing or debugging.
The main takeaway? Always declare your variables and functions before using them. It makes your code clearer and less error-prone.
Basically that's it.
The above is the detailed content of What is hoisting in JavaScript and how does it work?. For more information, please follow other related articles on the PHP Chinese website!
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