TypeScript 2.x By Example

Union types

Most of the time, we are aware of what type of data a specific variable can hold, and we can annotate the said variable with that type. This helps in type checking at compile time, and makes sure we don't misuse the variable. But there are times when a variable may not be confined to one specific type, but can have multiple types of values. This is common in scenarios where we are migrating JavaScript code, or using reference of JavaScript libraries. 

TypeScript has the union type to solve this problem. A union type allows us to define a variable with multiple types; for example, a variable can have number and string types. This is achieved by using the pipe (|) symbol between the types, as shown here:

let data : string | number; ...