34. Union Types

A Union Type allows a variable to be one of several types. This is useful when a value could be more than one type, and you need to handle each type differently.

Example:

In the padLeft function, the padding argument is defined as either a string or a number.

function padLeft(value: string, padding: string | number) {
  if (typeof padding === "number") {
    return Array(padding + 1).join(" ") + value;
  }
  if (typeof padding === "string") {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}
 
console.log(padLeft("Hello world", 4));  // returns "    Hello world"

The example shows how string | number is a union type, and TypeScript helps check which type the variable is at compile-time.