What is the difference between let and var in Swift?

Core answer

The simplest rule is:

• let creates a constant.
• var creates a variable.

With let, you cannot assign a new value later. With var, you can.

The interesting part is that this behaves differently for value types and reference types.

1. Value types: let makes the whole value read-only

Structs, enums, tuples, String, Array, and Dictionary are value types.

When you declare a value type with let, you cannot change any part of it, even if some properties were declared with var.

struct Counter {
    var count: Int
}

let fixedCounter = Counter(count: 0)
fixedCounter.count += 1 // ❌ Error

var changingCounter = Counter(count: 0)
changingCounter.count += 1 // ✅ Works

Why? Because changing a value type means changing the value itself. If it's declared with let, Swift doesn't allow that.

A simple interview rule is:

For value types, let makes the entire value immutable.

2. Reference types: let only locks the reference

Classes are reference types.

When you write let user = User(...), user always points to the same object. However, the object itself can still change if it has mutable (var) properties.

final class User {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let user = User(name: "Ana")
user.name = "Priya" // ✅ Allowed

What you cannot do is make user point to another object.

final class User {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let user = User(name: "Ana")
user = User(name: "Priya") // ❌ Error

A simple interview rule is:

For classes, let locks the reference, not the object.

Interview takeaway

Remember these three rules:

• let creates a constant, var creates a variable.
• For value types, let makes the whole value immutable.
• For reference types, let prevents changing the reference, but mutable (var) properties can still be modified.