JavaScript的變量和函數(shù)聲明會被提升到作用域頂部，但行為不同。 1. 函數(shù)聲明整體被提升，可在定義前調(diào)用；2. var聲明的變量僅提升聲明部分，賦值留在原地，導(dǎo)致訪問時可能為undefined；3. let和const不會被提升，存在暫時性死區(qū)，提前使用會報錯；4. 類聲明類似let/const，也受TDZ影響，而類表達(dá)式則像函數(shù)表??達(dá)式一樣處理；5. 實踐中建議將聲明置於作用域頂部，優(yōu)先使用let/const，並避免依賴提昇機(jī)制。

Hoisting in JavaScript might sound fancy, but it's actually a pretty straightforward concept once you break it down. The short version is: JavaScript moves variable and function declarations to the top of their scope before code execution . That's why sometimes you can use a function or variable before declaring it — the interpreter already knows they exist.

Let's dig into how this works with different types of declarations.

Function Declarations Are Fully Hoisted

When you define a function like this:

 sayHello();

function sayHello() {
  console.log("Hello!");
}

It works fine, even though we called sayHello() before writing the function. That's because the entire function declaration is hoisted , including its body. The interpreter sees that function as being declared at the top of the current scope (either global or local).

This behavior makes function declarations very forgiving when it comes to ordering your code.

Variable Hoisting Only Applies to Declarations, Not Initializations

Variables declared with var are also hoisted — but only the declaration, not the value. So this:

 console.log(myName);
var myName = "Alice";

Will log undefined , not an error. What happens is:

• var myName; gets moved to the top
• Then later, the assignment myName = "Alice"; runs
• But during the console.log , it hasn't been assigned yet

So the variable exists, but has no value until the actual line where you assign it.

If you're using let or const , things change — they aren't accessible before the declaration line. Trying to use them early will throw a ReferenceError . This area between the start of the scope and the actual declaration is known as the Temporal Dead Zone (TDZ) .

Class Declarations vs Expressions

Similar to variables, classes have different hoisting behaviors depending on how you define them.

A class declaration is hoisted, but just like let and const , it's not available until the line it's declared on.

 new Car(); // ReferenceError

class Car {}

But if you do something like:

 const myCar = new Car(); // ReferenceError
class Car {}

Still won't work because of the TDZ again.

On the flip side, class expressions (especially anonymous ones) behave more like function expressions — the identifier isn't hoisted unless explicitly assigned.

Practical Tips for Dealing With Hoisting

Here are a few real-world takeaways:

• ? Always declare functions and variables at the top of their scope if you want to avoid confusion.
• ? Prefer let and const over var — they make hoisting-related bugs less likely.
• ? Don't rely on hoisting in production code — even if it works, it can be confusing to others reading your code.
• ? When debugging, remember: just because it looks like a variable exists doesn't mean it's initialized yet.

You don't need to memorize all the rules — just know that hoisting exists, and understand the difference between declarations and assignments.

That's basically how hoisting works in JS — simple, but easy to misuse if you're not careful.

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