String Literal Types

String literal types allow you to specify the exact value a string must have. In practice string literal types combine nicely with union types, type guards, and type aliases. You can use these features together to get enum-like behavior with strings.

type Easing = "ease-in" | "ease-out" | "ease-in-out";
class UIElement {
  animate(dx: number, dy: number, easing: Easing) {
    if (easing === "ease-in") {
      // ...
    }
    else if (easing === "ease-out") {
    }
    else if (easing === "ease-in-out") {
    }
    else {
      // error! should not pass null or undefined.
    }
  }
}
 
let button = new UIElement();
button.animate(0, 0, "ease-in");
button.animate(0, 0, "uneasy"); // error: "uneasy" is not allowed here

You can pass any of the three allowed strings, but any other string will give the error

Argument of type '"uneasy"' is not assignable to parameter of type '"ease-in" | "ease-out" | "ease-in-out"'

String literal types can be used in the same way to distinguish overloads:

function createElement(tagName: "img"): HTMLImageElement;
function createElement(tagName: "input"): HTMLInputElement;
// ... more overloads ...
function createElement(tagName: string): Element {
  // ... code goes here ...
}

Links:

https://www.typescriptlang.org/docs/handbook/advanced-types.html#string-literal-types

doc_TypeScript

2025-01-10 15:47:30

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