const is preferred by default as it prevents reassignment and signals intent clearly; 2. use let when reassignment is needed, such as in loops or changing state; 3. avoid var due to its function-scoping, hoisting issues, and potential for bugs; 4. let and const are block-scoped and hoisted in a Temporal Dead Zone, making them safer and more predictable than var; 5. const does not make objects or arrays immutable—only the binding is protected, so properties can still be modified—all these differences ensure cleaner, more maintainable code when followed properly.

When writing JavaScript, you’ll often need to declare variables — and you’ve got three keywords to choose from: var, let, and const. While they all serve the purpose of storing values, they behave very differently under the hood. Understanding these differences is crucial for writing predictable, bug-free code.

Let’s break down each one and explore their scoping, hoisting behavior, reassignment rules, and real-world implications.

1. Scope: Where Your Variables Live

The most important difference between var, let, and const is scope — that is, where in your code a variable can be accessed.

• var is function-scoped
  If you declare a variable with var inside a function, it’s only accessible within that function. But if you declare it inside a block (like an if statement or for loop), it’s still accessible outside the block — as long as it’s in the same function or global scope.

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

• let and const are block-scoped
  They only exist within the block (anything inside {}) where they’re defined.

  

  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 much safer — they prevent accidental access from outside the intended scope.

2. Hoisting: How Variables Are Lifted During Compilation

JavaScript moves all variable declarations to the top of their scope during the compilation phase — this is called hoisting. But how each keyword behaves when hoisted is different.

• var is hoisted and initialized with undefined
  You can access a var variable before it’s declared — but it will be undefined.

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

  This can lead to confusing bugs because the variable exists but holds no meaningful value yet.

• let and const are hoisted but not initialized
  They exist in a special zone called the Temporal Dead Zone (TDZ) until their declaration is reached. Accessing them before declaration throws a ReferenceError.

  console.log(b); // ReferenceError: Cannot access 'b' before initialization
  let b = 5;

  This enforces better coding practices by preventing the use of variables before they’re declared.

3. Reassignment and Mutability

This is where let and const differ in intent, even though both are block-scoped.

• let allows reassignment
  You can change the value of a let variable as many times as needed.

  let count = 1;
  count = 2; // ? OK

• const does not allow reassignment
  Once you assign a value to a const, you can’t assign a new one.

  const name = "Alice";
  name = "Bob"; // TypeError: Assignment to constant variable

  ?? Important: const doesn’t make the value immutable — it only protects the assignment. If the value is an object or array, its contents can still be modified.

  const user = { name: "Alice" };
  user.name = "Bob"; // ? OK — object property changed
  user.age = 25;     // ? OK — new property added
  
  user = {}; // ? TypeError — can't reassign the variable

So const means “this variable will always point to the same value,” not “the value itself is frozen.”

4. When to Use Which? Best Practices

Now that we’ve seen the technical differences, here’s how to use them effectively:

• Use const by default
  If you don’t plan to reassign the variable (which is most cases), use const. It makes your intent clear and prevents accidental reassignment.

• Use let when you need to reassign
  Loop counters, accumulators, or flags that change state are good candidates.

  for (let i = 0; i < 10; i  ) { ... }

• Avoid var in modern JavaScript
  It’s outdated in most contexts due to its loose scoping and hoisting quirks. The only reason to use var today is if you're working in an old environment that doesn’t support let/const (very rare now).

Summary of Key Differences

? Note: Redeclaring a variable with var in the same scope is allowed (though dangerous), but let and const will throw an error if you try to redeclare.

Basically, stick with const unless you need reassignment — then use let. Forget var. It’s that simple.

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