var是函數(shù)作用域且可被重新賦值，let和const是塊作用域，其中l(wèi)et可重新賦值，const不可重新賦值但可修改引用對象的內(nèi)容；var聲明會提升並初始化為undefined，let和const提升但不初始化，存在暫時性死區(qū)；var在全局作用域下會成為window對象的屬性，而let和const不會；建議優(yōu)先使用const，需要重新賦值時使用let，避免使用var。

In modern JavaScript (ES6 ), var , let , and const are all used to declare variables, but they behave quite differently in terms of scope, hoisting, and reassignment. Understanding these differences is key to writing clean, bug-free code.

Here's a breakdown of how they differ:

1. Scope: Where Your Variables Live

The most important difference lies in scope — where a variable is accessible in your code.

• var is function-scoped
  Variables declared with var are only accessible within the function they're defined in. If declared outside any function, they're globally scoped. But here's the catch: var does not respect block scope (like inside if statements or for loops).

   if (true) {
    var x = 10;
  }
  console.log(x); // 10 — accessible outside the block!

• let and const are block-scoped
  They exist only within the nearest enclosing {} block.

  

   if (true) {
    let y = 20;
    const z = 30;
  }
  console.log(y); // ReferenceError: y is not defined
  console.log(z); // ReferenceError: z is not defined

This makes let and const safer and more predictable.

2. Reassignment: Can You Change the Value?

• var and let allow reassignment:

   var a = 1;
  a = 2; // OK
  
  let b = 1;
  b = 2; // OK

• const does not allow reassignment:

   const c = 1;
  c = 2; // TypeError: Assignment to constant variable

?? Note: const doesn't mean the value is immutable — it means the variable binding can't be changed. For objects and arrays, their contents can still be modified:

 const user = { name: 'Alice' };
user.name = 'Bob'; // This is fine
user.age = 25; // Also fine

// But this would fail:
user = {}; // TypeError: Assignment to constant variable

3. Hoisting: When Variables Are "Moved" to the Top

All three are hoisted (moved to the top of their scope during compilation), but they behave differently:

• var is hoisted and initialized with undefined .

   console.log(temp); // undefined, not an error
  var temp = "hello";

  This can lead to bugs because you can access var variables before they're declared.

• let and const are hoisted but not initialized . Accessing them before declaration causes a ReferenceError . This period is called the Temporal Dead Zone (TDZ) .

   console.log(name); // ReferenceError
  let name = "John";

This enforces better coding practices — you must declare before using.

4. Global Object Binding

In the global scope (outside functions, in the browser):

• var creates a property on the global window object:

   var apiKey = '123';
  console.log(window.apiKey); // '123'

• let and const do not add properties to window :

   let apiKey = '123';
  console.log(window.apiKey); // undefined

This helps avoid accidental global pollution.

Which Should You Use?

• ? Use const by default
  If you don't plan to reassign the variable (most cases), use const . It's clearer and prevents accidental changes.

• ? Use let when you need to reassign
  Like in loops or counters:

   let count = 0;
  count ;

• ? Avoid var in modern code
  It's outdated due to its confusing scoping and hoisting behavior. Use let or const instead.

Basically, stick with const and let , and you'll write safer, more predictable JavaScript.

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