Union Types in TypeScript

In TypeScript you can use union types to describe values that can be of more than one type. It therefore allows to avoid using any. Define union types with the | character to separate the different possible types.

For example, let’s say a function definition uses parameters that describe values that can be either strings or numbers. Instead of this:

function add(v1: any, v2: any) {
  let value1 = typeof v1 === "string" ? +v1 : v1;
  let value2 = typeof v2 === "string" ? +v2 : v2;

  console.log(value1 + value2);
}

add(23, "32"); // 55
add(23, true); // No error when passing non-string or number value

You can do this:

function add(v1: number | string, v2: number | string) {
  let value1 = typeof v1 === "string" ? +v1 : v1;
  let value2 = typeof v2 === "string" ? +v2 : v2;

  console.log(value1 + value2);
}

add(23, "32"); // 55
add(23, true); // Error when passing non-string or number value