Union Types in TypeScript

Published on November 28, 2016

TypeScript

By Alligator.io

Developer and author at DigitalOcean.

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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