Day12: Learning TypeScript: Literal Types / Tuple

Literal Types (字面值型別)

String Literal Types

String literal types are a way to represent specific values as types, restricting the value to one of a few specific strings. For example, we can make the literal value of variable x equal to "hello".

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

But it is rare to have a variable with only one value. We can combine desired values, such as giving alignment three literal values, where the parameter must match one of them. If it does not match one of these three values or is misspelled, it will throw an error.

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

Numeric Literal Types

Similarly, numeric operations can also be restricted:

The return value of the compare function can only be -1, 0, or 1.

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

Non-Literal Types

Non-literal types can also be combined:

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

Tuples (元組)

Tuples are a combination of different types of objects. For example, we can define a tuple with a string and a number:

const iris: [string, number] = ['iris', 18];

You can use and assign values to the properties of the types. However, when initializing or assigning a tuple type variable, all items must be provided. Adding items of different types is also not allowed:

let tom: [string, number];
tom = ['tom', 18]; // If tom is only declared without assignment, it will be undefined, and if tsconfig strictNullChecks is enabled, it will throw an error.
tom[0] = 'Tom'; //ok
tom[1] = 25; //ok
tom[0].slice(1); //ok
tom[1].toFixed(2); //ok
tom.push('male'); //ok

tom = ['tom chen']; //error: Property '1' is missing in type '[string]' but required in type '[string, number]'.
tom.push(true); //error: Argument of type 'true' is not assignable to parameter of type 'string | number'.

This is a test example while learning, feel free to refer here.
Thank you for reading, see you tomorrow!

References

https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types https://willh.gitbook.io/typescript-tutorial/advanced/tuple