Difference between const and readonly in typescript

A const variable cannot be re-assigned, just like a readonly property.

Essentially, when you define a property, you can use readonly to prevent re-assignment. This is actually only a compile-time check.

When you define a const variable (and target a more recent version of JavaScript to preserve const in the output), the check is also made at runtime.

So they effectively both do the same thing, but one is for variables and the other is for properties.

const x = 5;

// Not allowed
x = 7;


class Example {
    public readonly y = 6;
}

var e = new Example();

// Not allowed
e.y = 4;

Important note... "cannot be re-assigned" is not the same as immutability.

const myArr = [1, 2, 3];

// Not allowed
myArr = [4, 5, 6]

// Perfectly fine
myArr.push(4);

// Perfectly fine
myArr[0] = 9;

both:

• can be changed (for example by .push() if array)

const:

• can't be reassigned
• used for variables

readonly:

• can be reassigned but only inside constructor
• used for properties (class member)

1. I think that the accepted answer didn't emphasized enough that const is expected to be used with variables while readonly with class/interface properties.

2. readonly is checked only during type-checking (compile time) while const is checked during runtime

3. declaring a property to be readonly doesn't mean that its value can't be changed: it means that the property cannot be re-assigned, example:

interface Person {
    readonly info: { name: string; age: number };
}

//create a new person
// ...

person.info.age += 1; // this is valid
person.info = { name: "Johnny", age: 15 }; // this is invalid!

4. Since TS 3.4 there are const assertions which are used to define types:

// Type 'readonly [10, 20]'
let y = [10, 20] as const;

// Type '{ readonly text: "hello" }'
let z = { text: "hello" } as const;

For more, see the docs

as said in https://www.typescriptlang.org/docs/handbook/interfaces.html#readonly-vs-const

The easiest way to remember whether to use readonly or const is to ask whether you’re using it on a variable or a property. Variables use const whereas properties use readonly.

as it displayed in below image if you declare const in defining a property you will get an error https://i.imgur.com/cJfDqh9.png

I think the reason for both words is probably that it's easier to implement than the alternative.

In C++, for instance, classes can have const members. But C++ has special syntax to initialize the constants before the constructor is run, so there's never really any "assignment" to the const going on, like:

class TestClass {
    const int _x;
    TestClass(int x) : _x(x) {}
}

The _x(x) initializes the _x variable before the constructor is called.

In Typescript, if they wanted to allow members to be declared const and were serious about the variables being const, they would need to add similar syntax.

"readonly" isn't really the same as const. It means, I think, something slightly different. It means "allow assignment inside the constructor, but then no more."

const

1. constant in compile time
2. must be initialized on its declaration
3. can be declared in methods

readonly

1. constant in execution time
2. don't need to be initialized on its declaration
3. can't be declared in methods, only constructor or self declaration