Literal Types

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Literal

In addition to the general types string and number, we can refer to specific strings and numbers in type positions.

One way to think about this is to consider how JavaScript comes with different ways to declare a variable. Both var and let allow for changing what is held inside the variable, and const does not. This is reflected in how TypeScript creates types for literals.

let changingString = "Hello World";
changingString = "Olá Mundo";
// Because `changingString` can represent any possible string, that
// is how TypeScript describes it in the type system
changingString;

let changingString: string

const constantString = "Hello World";
// Because `constantString` can only represent 1 possible string, it
// has a literal type representation
constantString;

const constantString: "Hello World"

let x: "hello" = "hello";
// OK
x = "hello";
// ...
x = "howdy";
Type '"howdy"' is not assignable to type '"hello"'.

But by combining literals into unions, you can express a much more useful concept - for example, functions that only accept a certain set of known values:

function printText(s: string, alignment: "left" | "right" | "center") {
  // ...
}
printText("Hello, world", "left");
printText("G'day, mate", "centre");
Argument of type '"centre"' is not assignable to parameter of type '"left" | "right" | "center"'.

Numeric literal types work the same way:

function compare(a: string, b: string): -1 | 0 | 1 {
  return a === b ? 0 : a > b ? 1 : -1;
}

function compare must return -1,0 or 1

interface Options {
  width: number;
}
function configure(x: Options | "auto") {
  // ...
}
configure({ width: 100 });
configure("auto");
configure("automatic");
Argument of type '"automatic"' is not assignable to parameter of type 'Options | "auto"'.