TypeScript has a specific syntax for typing function parameters and return values.
Read more about functions here .
Return Type
The type of the value returned by the function can be explicitly defined.
Example
// the `: number` here specifies that this function returns a number function getTime(): number {
return new Date().getTime();
}If no return type is defined, TypeScript will attempt to infer it through the types of the variables or expressions returned.
Void Return Type
The type void can be used to indicate a function doesn't return any value.
Example
function printHello(): void {
console.log('Hello!');
}Parameters
Function parameters are typed with a similar syntax as variable declarations.
Example
function multiply(a: number, b: number) {
return a * b;
}If no parameter type is defined, TypeScript will default to using any , unless additional type information is available as shown in the Default Parameters and Type Alias sections below.
Optional Parameters
By default TypeScript will assume all parameters are required, but they can be explicitly marked as optional.
Example
// the `?` operator here marks parameter `c` as optional function add(a: number, b: number, c?: number) {
return a + b + (c || 0);
}Default Parameters
For parameters with default values, the default value goes after the type annotation:
Example
function pow(value: number, exponent: number = 10) {
return value ** exponent;
}TypeScript can also infer the type from the default value.
Named Parameters
Typing named parameters follows the same pattern as typing normal parameters.
Example
function divide({ dividend, divisor }: { dividend: number, divisor: number }) {
return dividend / divisor;
}Rest Parameters
Rest parameters can be typed like normal parameters, but the type must be an array as rest parameters are always arrays.
Example
function add(a: number, b: number, ...rest: number[]) {
return a + b + rest.reduce((p, c) => p + c, 0);
}Type Alias
Function types can be specified separately from functions with type aliases.
These types are written similarly to arrow functions, read more about arrow functions here .
Example
type Negate = (value: number) => number;
// in this function, the parameter `value` automatically gets assigned the type `number` from the type `Negate`
const negateFunction: Negate = (value) => value * -1;