TypeScript mapped type example

TypeScript Advance Types: Exercise-12 with Solution

Write a TypeScript program that defines a mapped type. It takes an object type as a parameter and makes all its properties read-only (immutable). Test the mapped type with an example object.

Sample Solution:

TypeScript Code:

// Define an example object type with properties
type Food = {
  name: string;
  price: number;

// Define a mapped type 'ReadOnly' that makes all properties read-only
type ReadOnly<T> = {
  readonly [K in keyof T]: T[K];

// Create an example object of type 'Food'
const product: Food = {
  name: "italian pizza",
  price: 6, // In €

// Use the 'ReadOnly' mapped type to make the properties read-only
type ReadOnlyProduct = ReadOnly<Food>;

// Create an instance of the 'ReadOnlyProduct' type
const readOnlyFood: ReadOnlyFood = {
  name: "American burger",  
  price: 4.79,     // In US $

// Attempt to modify a property (will result in a TypeScript error)
// readOnlyProduct.name = "American burger";  // Error: Cannot assign to 'name' because it is a read-only property

// Print the 'readOnlyProduct' object


In the exercise above -

  • First, we define an example object type called "Food" with two properties: 'name' and 'price'.
  • Next, we define a mapped type called "ReadOnly" that takes a generic type parameter T. Within the mapped type, we use a mapped type transformation to iterate over the keys (K) of the input type (T). For each property, we use the readonly modifier to make it read-only.
  • Create an instance of the "Item" type called "item".
  • Use the ReadOnly mapped type to create a new type called "ReadOnlyProduct", which makes all properties read-only.
  • Create an instance of the ReadOnlyFood type called 'readOnlyFood'. You can see that we've provided values for both properties, and they are now read-only.
  • Finally we attempt to modify a property of "readOnlyFood", which results in a TypeScript error because read-only properties cannot be modified.


Uncaught ReferenceError: readOnlyProduct is not defined

TypeScript Editor:

See the Pen TypeScript by w3resource (@w3resource) on CodePen.

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