Mastering TypeScript 3 - Third Edition

Union types

TypeScript allows us to express a type as the combination of two or more other types. These types are known as union types, and their declaration uses the pipe symbol (|) to list all of the types that will make up the new type. Consider the following TypeScript code:

var unionType : string | number; 
 
unionType = 1; 
console.log(`unionType : ${unionType}`); 
 
unionType = "test"; 
console.log(`unionType : ${unionType}`); 

Here, we have defined a variable named unionType, which uses the union type syntax to denote that it can hold either a string or a number. We are then assigning a number to this variable, and logging its value to the console. We then assign a string to this variable, and again log its value to the console. This code ...