In TypeScript, you can define read-only properties using the `readonly` modifier. When you mark a property as `readonly`, it means that once the property is set with a value, it cannot be changed or reassigned. Here's how you can define read-only properties in TypeScript: 

class MyClass {
  readonly readOnlyProperty: number;

  constructor(value: number) {
    this.readOnlyProperty = value;
  }

  // You can have other methods and properties in the class as well
}

const instance = new MyClass(10);

console.log(instance.readOnlyProperty); // Output: 10

// You cannot reassign the read-only property once it's set
// instance.readOnlyProperty = 20; // This will result in a TypeScript error
In the example above, the `readOnlyProperty` is marked as `readonly`, and its value is set through the constructor. Once it is set, you cannot modify its value anymore. Note that the `readonly` modifier applies only to the property itself, not to the properties of an object that the property might refer to. If the read-only property holds a reference to an object, the properties of that object can still be modified. To make the properties of the object read-only as well, you need to apply the `readonly` modifier to each property within that object. Here's an example of a read-only property containing a mutable object: 

class MyClass {
  readonly mutableObject: { value: number };

  constructor(value: number) {
    this.mutableObject = { value };
  }

  // You can have other methods and properties in the class as well
}

const instance = new MyClass(10);

console.log(instance.mutableObject.value); // Output: 10

// While the mutableObject itself is read-only, the properties of the object can still be modified
instance.mutableObject.value = 20;
console.log(instance.mutableObject.value); // Output: 20
To make the properties of `mutableObject` also read-only, you would need to use TypeScript's `Readonly` utility type: 

class MyClass {
  readonly mutableObject: Readonly<{ value: number }>;

  constructor(value: number) {
    this.mutableObject = { value };
  }

  // You can have other methods and properties in the class as well
}

const instance = new MyClass(10);

console.log(instance.mutableObject.value); // Output: 10

// Now the attempt to modify the property will result in a TypeScript error
// instance.mutableObject.value = 20; // TypeScript error: Cannot assign to 'value' because it is a read-only property.
With this approach, both the `mutableObject` property itself and its properties are read-only, preventing any modifications after instantiation.