Readonly
The readonly keyword is used to define properties whose values can not be changed once they are initialized with a value. The readonly is mostly used with the properties of objects, classes and interfaces.
Syntax:
class className {
public readonly propertyName = value;
}
Features:
- A property defined using readonly must be assigned at the time of declaration or using the constructor function of the class (if it’s a class property).
- The value of readonly property can not be re-assigned with a new value.
- The readonly can be used to create a entire object or specific properties as readonly.
Example: The below code example illustrates the practical implementation of readonly to define properties.
Javascript
// Implementation with classesclass cmpny { public readonly name: string = "w3wiki"; public readonly desc: string = "A Computer Science Portal.";}const myCmpny = new cmpny();console.log (`Company: ${myCmpny.name}, Description: ${myCmpny.desc}`);myCmpny.name = "New Company";myCmpny.desc = "New Description";console.log (`Company: ${myCmpny.name}, Description: ${myCmpny.desc}`);// Implementation with interface and objectinterface readonlyInterface { readonly crktr_name: string, readonly runs: number}const obj: readonlyInterface = { crktr_name: "Virat Kohli", runs: 26000}console.log (`Cricketer Name: ${obj.crktr_name}, Runs: ${obj.runs}`);obj.crktr_name = "Sachin Tendulkar";obj.runs = 30000;console.log (`Cricketer Name: ${obj.crktr_name}, Runs: ${obj.runs}`); |
Output:
Company: w3wiki, Description: A Computer Science Portal.
Cannot assign to 'name' because it is a read-only property.
Cannot assign to 'desc' because it is a read-only property.
Cricketer Name: Virat Kohli, Runs: 26000
Cannot assign to 'crktr_name' because it is a read-only property.
Cannot assign to 'runs' because it is a read-only property.
Difference Between Const and Readonly in TypeScript
The const and readonly both are used to create the variables and properties that can not be re-assigned once they get initialized by some value. But they are different from each other. Let us discuss the differences between them.