let myFavoriteNumber: string | number;
myFavoriteNumber = 'seven';
myFavoriteNumber = 7;
聯合型別會用 |
來分隔每個型別
function getLength(something: string | number): number {
return something.length;
}
// index.ts(2,22): error TS2339: Property 'length' does not exist on type 'string | number'.
// Property 'length' does not exist on type 'number'.
.toString()
string 和 number 都能使用,所以沒有報錯
function getString(something: string | number): string {
return something.toString();
}
let myFavoriteNumber: string | number;
myFavoriteNumber = 'seven';
console.log(myFavoriteNumber.length); // 5
myFavoriteNumber = 7;
console.log(myFavoriteNumber.length); // 編譯時報錯
// index.ts(5,30): error TS2339: Property 'length' does not exist on type 'number'.
or
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