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Amit Merchant

Amit Merchant

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Emulate enum-like behavior using string literal types in TypeScript

Sometimes, all you want to do is a custom type populated with a set of predefined strings which can be used to restrict a variable to only have values from this set of strings.

In TypeScript, this can be done by using “string literal types” which allow you to specify the exact value a string must have. In other words, string literal types can be used to create enum-like behavior without adding many complexities.

So, for instance, if you want to make a string literal type of the post status, here’s how you can do so.

type PostStatus = "draft" | "published" | "archived";

As you can see, the type is composed using nothing but a unionization of certain string values. In this case, different statuses of a post.

Now, you can apply this type to a variable like so.

class Post {
    setStatus(status: PostStatus): void
        if (status === "draft") {
            // ...
        } else if (easing === "published") {
        } else if (easing === "archived") {
        } else {
            // error! should not pass null or undefined.

let post = new Post();
post.setStatus("canceled"); // error: "canceled" is not allowed here

Here, the method can take in any of the three allowed strings (draft, published, archived) but any string other than these will give the following error.

Argument of type '"canceled"' is not assignable to parameter of type '"draft" | "published" | "archived"'

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