What is the "union" type in TypeScript?

The 'union' type in TypeScript allows a variable to store one of several types. It is a feature that enhances JavaScript's static typing capabilities. In JavaScript, variables can hold any type of data, whereas TypeScript's union type enables explicit specification of which types a variable can accept.

For example, we can define a variable that can store either string or number types:

typescript
let myVariable: string | number;

myVariable = "Hello, World!"; // Valid assignment
myVariable = 42;              // Also valid

// When attempting to assign other types, TypeScript will throw an error
// myVariable = true;         // Error: Type 'boolean' is not assignable to type 'string | number'.

The 'union' type is particularly useful when handling inputs that may be of multiple types. For instance, when processing user input or function return values from third-party libraries, these values can represent one of several types. By utilizing union types, we can ensure correct type checking at compile time, thereby enhancing the robustness and safety of the code.