在JavaScript中，var、let和const的主要區(qū)別在於作用域、提升行為和可重新賦值性。 var是函數(shù)作用域，而let和const為塊作用域；var聲明的變量會被提升並初始化為undefined，而let和const雖被提升但不會初始化，訪問會報(bào)錯(cuò)（暫時(shí)性死區(qū)）；var和let允許重新賦值，const不允許重新賦值，但可變對象的內(nèi)容可修改。使用時(shí)應(yīng)默認(rèn)優(yōu)先用const，需要重新賦值時(shí)用let，避免使用var。

In JavaScript, var , let , and const are all used to declare variables, but they behave differently in terms of scope, hoisting, and reassignment. Understanding these differences is essential for writing clean and bug-free code.

Function Scope vs. Block Scope

The main difference between var and let / const lies in their scope .

• var is function-scoped , meaning a variable declared with var is accessible throughout the function it's declared in — not just the block where it was created.

  Example:

  

   if (true) {
    var x = 10;
  }
  console.log(x); // Outputs: 10

• let and const are block-scoped , so they're only accessible within the block {} they were declared in.

  Example:

   if (true) {
    let y = 20;
  }
  console.log(y); // ReferenceError

This makes let and const safer to use because they prevent accidental access or modification from outside the intended scope.

Hoisting Behavior

All three declarations are hoisted, but they behave differently:

• Variables declared with var are hoisted and initialized with undefined , which means you can access them before declaration (though the value will be undefined ).

  Example:

   console.log(a); // undefined
  var a = 5;

• let and const are also hoisted, but not initialized . Accessing them before declaration results in a ReferenceError . This period is known as the Temporal Dead Zone (TDZ) .

  Example:

   console.log(b); // ReferenceError
  let b = 10;

So, even though all three are hoisted, using let and const encourages better coding practices by enforcing variable declaration before usage.

Reassignment and Mutability

Here's how each behaves when it comes to changing values:

• var and let allow reassignment :

   let name = "Alice";
  name = "Bob"; // Allowed

• const does not allow reassignment , but the content of reference types like objects and arrays can still be changed:

   const arr = [1, 2];
  arr.push(3); // Allowed
  arr = []; // TypeError: Assignment to constant variable.

So const doesn't make values immutable, just prevents reassigning the variable identifier.

When to Use Which?

A few practical guidelines:

• Use const by default unless you know the variable needs to be reassigned.
• Use let when you need to reassign values, such as in loops or counters.
• Avoid var in modern JS unless you're maintaining legacy code.

You'll rarely have a reason to use var anymore since let and const provide more predictable behavior.

That's the core of how var , let , and const differ. The key takeaways are around scope, hoisting, and mutability — and knowing when to choose one over the others helps avoid bugs and confusion.

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