递归 function 在 Promise 中不起作用

Question

I have a function which has nested Promises within it. I want to re-execute this function when there is a length value of streamRes.length . Currently, it only executes the code and prints out the length on the console but doesn't re-execute.

let fetchEnrolleesData = () => {
    getLastUpdatedEnrollee().then(res => {
        let params = path+"enrollees?limit=100";
        params += res.last_date ? "&last_updated_at=" + res.last_date.value : '';

        fetchEnrolleesDataInStream(res, params).then(streamRes => {
            if(streamRes.length) {
                console.log(streamRes.length);
                fetchEnrolleesData();
            }
        });
    });
}

Answer 1

As evolutionxbox wrote , you need to return the promise returned by getLastUpdatedEnrollee as well as the promise getLastUpdatedEnrollee itself. In general, when calling a promise within a function, you need to return those promises in order to use those values outside of that function.

Here is your code with return added as needed:

let fetchEnrolleesData = () => {
    return getLastUpdatedEnrollee().then(res => {
        let params = path+"enrollees?limit=100";
        params += res.last_date ? "&last_updated_at=" + res.last_date.value : '';

        return fetchEnrolleesDataInStream(res, params).then(streamRes => {
            if(streamRes.length) {
                console.log(streamRes.length);
                return fetchEnrolleesData();
            }
        });
    });
}

As an optional side note, you may or may not prefer to use () => … syntax instead of () => { … } to remove the need to write return in the top-level function. You can do that in the top-level function because it has no other statements; this wouldn't work in the inner function, which has multiple statements. See arrow function expressions for more detail.

let fetchEnrolleesData = () =>
    getLastUpdatedEnrollee().then(res => {
        // …
    });