状態名と変数を束ねて型で制約

code:ts

type STATE_TYPE = "INITIAL" | "HAS_NUMBER";

let state: STATE_TYPE = "INITIAL";

let value: number | undefined;

const setNumber = (x: number) => {

state = "HAS_NUMBER";

value = x;

}

const useNumber = () => {

if (state === "HAS_NUMBER") {

const x: number = value; // NG

// Type 'number | undefined' is not assignable to type 'number'.

}

}

code:ts

const useNumber = () => {

if (state === "HAS_NUMBER") {

if (value !== undefined) { // mmm...

const x: number = value; // OK

}

}

}

code:ts

type STATE_TYPE = (

{ type: "INITIAL", value: undefined } |

{ type: "HAS_NUMBER", value: number }

);

let state: STATE_TYPE = { type: "INITIAL", value: undefined };

const setNumber = (x: number) => {

state = { type: "HAS_NUMBER", value: x };

}

const useNumber = () => {

if (state.type === "HAS_NUMBER") {

const x: number = state.value; // OK

}

}