Learn React with TypeScript 3

Using the typeof keyword

The typeof keyword is a JavaScript keyword that returns a string that represents the type. So, we can use this in a condition to narrow down the type.

Let's go through an example:

1. We have a union type that can be a string or an array of strings:

type StringOrStringArray = string | string[];

2. We need to implement a function called first that takes in a parameter of type StringOrStringArray and returns a string:

function first(stringOrArray: StringOrStringArray): string {  }

3. The function needs to return the first character if stringOrArray is a string; otherwise, it should return the first array element:

function first(stringOrArray: StringOrStringArray): string {  if (typeof stringOrArray === "string") { return ...