Porting Promise Chaining from AngularJs to VueJs

Similar to my last question but different enough that I am at a loss. I have the following function in AngularJs that I need to recreate in VueJs. I have two similar ways I’ve tried to write this in VueJs, but they are causing lots of site exceptions both ways.

AngularJs

var foo = function(obj, config) {
    if (config.skip) {
        return $q.reject("Skipping");
    }
    var deferred = $q.defer();
    obj.promise = deferred.promise;
    if (obj.hasValue()) {
        deferred.resolve(obj);
    } else {
        "/api/callToApi".$promise.then(function(res) {
            if (res) {
                deferred.resolve(res);
            else {
                deferred.reject(res);
            }
         });
    }
    return deferred.promise;
}

VueJs – take 1. I’m pretty sure this one is missing the actual promise chaining, not sure how to correctly set that.

var foo = function(obj, config) {
    let returnEarly = false;
    let promise = new Promise((resolve, reject) => {
        returnEarly = true;
        reject("Skipping"):
    }
    if (returnEarly) {
        return promise;
    }
    obj.promise = promise;
    return new Promise((resolve, reject) => {
        if (obj.hasValue()) {
            resolve(obj);
        } else {
            axios.get("/api/callToApi").then(function(res) {
                if (res) {
                    resolve(res);
                } else {
                    reject(res);
                }
            }
        }
    }
}

Console errors with take 1

Uncaught (in promise) Error: Request failed with status code 404
    at XMLHttpRequest.__capture__.onreadystatechange

VueJs – take 2. I thought this way would return the correct chaining, but I get an error Timeout - Async callback was not invoked within the 5000ms timeout when running jest tests.

var foo = function(obj, config) {
    let returnEarly = false;
    let promise = new Promise((resolve, reject) => {
        returnEarly = true;
        reject("Skipping"):
    }
    if (returnEarly) {
        return promise;
    }
    obj.promise = promise;
    return promise.then(() => {
        return new Promise((resolve, reject) => {
            if (obj.hasValue()) {
                resolve(obj);
            } else {
                axios.get("/api/callToApi").then(function(res) {
                    if (res) {
                        resolve(res);
                    } else {
                        reject(res);
                    } 
                }
            }
        }
    }
}

Answer

This is plain JavaScript, not specific to Vue.

It’s generally unnecessary to assign a promise to obj.promise because it’s returned from the function.

Excessive use of new Promise is known as promise constructor antipattern. If there’s already a promise (Axios returns one), there’s no need to create a new one, this results in redundant and error-prone code (this can be the reason for Async callback was not invoked... error). In case a new promise needs to be created, there are shortcut Promise methods.

Should be something like:

function(obj, config) {
    if (config.skip) {
        return Promise.reject("Skipping");
    }

    if (obj.hasValue()) {
        return Promise.resolve(obj);
    } else {
        return axios("/api/callToApi").then(function(res) {
            if (res)
                return res;
            else 
                throw res;
            }
         });
    }
}

It’s a bad practice in general to make errors anything but Error object.

Also notice that Axios response object is always truthy, possibly needs to be res.data.

Will be more concise when written as async..await.