如何检查JavaScript函数是否返回Promise？

Question

说我有两个功能:

function f1() {
    return new Promise<any>((resolve, reject) => {
        resolve(true);
    });
}

function f2() {
}

我怎么知道是否f1会返回Promise并f2不会呢？

Answer 1

调用该函数,使用 instanceof

let possiblePromise = f1();
let isPromise = possiblePromise instanceof Promise;

Answer 2

所以这只是关于规范化输出.然后运行它Promise.resolve()和co.

var possiblePromise = f1();
var certainPromise = Promise.resolve(possiblePromise).then(...);

要么

var possiblePromises = [f1(), f2()];
Promise.all(possiblePromises).then(values => ...);

但是你需要对这些函数返回的基本概念有所了解.例如:两个例子都会失败

function f3(){
    return [somePromise, someOtherPromise];
}

var arrayOfPromisesOrValues = f3();

//here you'll still deal with an Array of Promises in then();
Promise.resolve(arrayOfPromisesOrValues).then(console.log); 
//neither does this one recognize the nested Promises
Promise.all([ f1(), f2(), f3() ]).then(console.log);

//only these will do:
Promise.all(f3()).then(console.log);
Promise.all([ f1(), f2(), Promise.all(f3()) ]).then(console.log);

您需要知道f3返回一个Promises或值数组并处理它.就像你需要知道f1并f2返回一个值或一个值的Promise.

结论:您需要具备函数返回的基本知识,以便能够通过正确的函数运行结果来解析promise.