状態名と変数を束ねて型で制約
tstype 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'.
}
} tsconst useNumber = () => {
if (state === "HAS_NUMBER") {
if (value !== undefined) { // mmm...
const x: number = value; // OK
}
}
}tstype 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
}
} console.log(state) become more useful"Engineer's way of creating knowledge" the English version of my book is now available on [Engineer's way of creating knowledge]
(C)NISHIO Hirokazu / Converted from [Scrapbox] at [Edit]