What are the differences between const and readonly in TypeScript?
Richard W
In TypeScript,const andreadonly are used to define variables or properties with different levels of immutability. While they may seem similar, there are important differences between them. Let's explore the distinctions:
1.const:
Theconst keyword is used to declare a variable that is a constant, meaning its value cannot be reassigned after it is initially assigned. Here are some key characteristics ofconst:
-const is used for variables declared with a value, such as primitive values or object references.
- The value of aconst variable is evaluated and assigned only once at the point of declaration.
-const variables must be assigned a value when they are declared.
- Attempting to reassign aconst variable or change its value will result in a compilation error.
Example:
typescript
const pi = 3.14159;
pi = 3.14; // Error: Cannot assign to 'pi' because it is a constant.
2.readonly:
Thereadonly modifier is used to define properties within classes or interfaces to indicate that their values cannot be modified after initialization. Here are some key characteristics ofreadonly:
-readonly can be used for properties within classes or interfaces.
-readonly properties can be assigned a value during object creation or within the constructor of a class.
- The value of areadonly property can only be set during initialization and cannot be changed afterwards.
-readonly properties are useful when you want to ensure that certain properties remain constant throughout the lifetime of an object.
Example:
typescript
class Circle {
readonly pi = 3.14159;
readonly radius: number;
constructor(radius: number) {
this.radius = radius;
}
calculateArea(): number {
return this.pi * this.radius * this.radius;
}
}
const circle = new Circle(5);
console.log(circle.calculateArea()); // Output: 78.53975
circle.radius = 10; // Error: Cannot assign to 'radius' because it is a read-only property.
In this example, thepi property is declared asreadonly to indicate that its value cannot be modified. Theradius property is alsoreadonly, but it can be assigned a value within the constructor of theCircle class.
To summarize:
-const is used for variables and indicates that their value cannot be reassigned after declaration.
-readonly is used for properties within classes or interfaces and signifies that their values cannot be modified after initialization.
By usingconst andreadonly appropriately, you can enforce immutability at different levels, providing better clarity and maintaining the integrity of your code.
