TypeScript Assignability of Functions
In this article, we will explore the concept of assignability of functions in TypeScript. Specifically, we will discuss how functions with a return type of void can be assigned to function types with other return types, including those that return values.
Understanding Assignability
In TypeScript, functions with a void return type are assignable to function types with different return types. This means you can assign a function that does not return a value (void) to a variable or parameter expecting a function that returns a different type of value. However, TypeScript does not check if the returned value is being used when performing such assignments.
Contextual Syntax vs. Literal Syntax
1. Contextual Syntax:
This allows assignability and does not produce errors.
type FunctionType = () => void;
const functionName: FunctionType = () => {
// Function body
};
2. Literal Syntax:
This will give an error if the function does not match the expected return type.
function functionName(parameter1: Type1, parameter2: Type2, ...): void {
// Function body
}
Parameters
- functionType: custom name for function type.
- functionName: The name of the function of the type defined.
- : void: Indicates the return type of void, meaning the function doesn't return a meaningful value.
Examples of TypeScript Assignability of Functions
Example 1: In this example, we assign a void function to a function returning a string value. When the return value of the f1 function is assigned to another variable, it retains the type void.
type voidFunc = () => void;
const f1: voidFunc = () => {
return "GeeksforGeeks";
};
const gfg = f1();
console.log(gfg)
console.log(typeof gfg)
Output:
Example 2: In this example, we will see even though Array.prototype.push returns a number and the Array.prototype.forEach method expects a function with a return type of void, the code is valid.
const gfg =
["Java","C++", "Python","React","Typescript"];
const course = [];
gfg.forEach((el) => course.push(el));
console.log(course)
Output:
