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TypeScript Functions


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();
}
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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!');
}

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Parameters

Function parameters are typed with a similar syntax as variable declarations.

Example

function multiply(a: number, b: number) {
  return a * b;
}
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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.


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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);
}
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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;
}
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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;
}
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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);
}
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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;
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TypeScript Exercises

Test Yourself With Exercises

Exercise:

Create a function that returns the string "Learning is Fun!", with the return type explicitly defined:

 myFunc():  {
   "Learning is Fun!";
}

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