const sum : (a: number, b: number) => number = (a, b) => a + b;
参考: https://typescript-jp.gitbook.io/deep-dive/type-system/callable#ar
const sum : (a: number, b: number) => number = (a, b) => a + b;
参考: https://typescript-jp.gitbook.io/deep-dive/type-system/callable#ar