TypeScript Working with Union Types

In this article, we are going to learn about Working with Union Types in Typescript. TypeScript is a popular programming language used for building scalable and robust applications. In Typescript, the compiler will only allow an operation if it is valid for every member of the union.

For example, if you have the union string | number, you can’t use methods that are only available on a string or only available on a number. To handle this problem, we need to use type guards/ type assertions to narrow down the type within the union before performing the operation.

Problems Arise While using the Union Type

Suppose, a function takes a parameter which is a union type, and wants to perform an operation that is specific to one of the types in the union. TypeScript may not allow this operation because it needs to ensure that the operation is valid for all possible types within the union.

Example: In this example, TypeScript raises an error because the length property is specific to strings, and it cannot guarantee that input is always a string when working with the union type string | number.

Javascript

function printLength(input: string | number) {
    console.log(input.length);

    // Error: Property 'length' does not exist
    // on type 'string | number'.
}

Output:

Solution of the Above Problem

To address this issue, we can use type assertions or type guards to narrow down the type within the union before performing the operation.

Example: We ae using typeof to do the narrowing for above example code.

Javascript

function printLength(input: string | number) {
    if (typeof input === 'string') {

        // OK, input is asserted as string type
        console.log(input.length);
    } else {
        console.log('Not a string');
    }
}

printLength("GeeksforGeeks")

Output:

Example 2: Sometimes you’ll have a union where all the members have something in common. For example, both arrays and strings have a slice method. If every member in a union has a property in common, you can use that property without narrowing:

Javascript

// Return type is inferred as number[] | string
function getFirstFour(x: number[] | string) {
    return x.slice(0, 4);
}

console.log(getFirstFour("GeeksforGeeks"))

Output: