Best way to use if/else in promise chains – JavaScript – SitePoint Forums

Hello everyone-

See this SO reply for the best way to include if/else in a promise chain. The accepted solution looks like this:

new Promise(resolve => resolve({success:true}))
.then(value => {
    console.log('second block:', value);
    if (value.success) {
        //skip the rest of this chain and return the value to caller
        return value;
    }
    //do something else and continue
    return somethingElse().then(value => {
        console.log('3rd block:', value);
        return value;
    });
}).then(value => {
    //The caller's chain would continue here whether 3rd block is skipped or not
    console.log('final block:', value);
    return value;
});

I’ve played around with it and this seems to work.

Note the syntax with two return remarks (interesting)

*return* somethingElse().then(value => {
    console.log('3rd block:', value);
    *return* value;
});

Is this the recommended way to embed if/else into js promise chains?

I found another old SO solution here, but this one seems to create too much unnecessary stuff function statement.

(Why are promises so hard to master? Don’t answer…)

I don’t understand why you use success in resolution returns? That’s why you have resolution and rejection. For me, resolve is called only if success is true, reject is called otherwise.

        return new Promise((resolve, reject) => {
            xhr.open(options.method || 'get', url, true);

            xhr.upload.addEventListener('progress', options.onProgress || (() => {
            }));

            xhr.addEventListener('load', () => {
                if (xhr.status >= 200 && xhr.status < 300) {
                    resolve(xhr.responseText);
                } else {
                    reject(new Error(xhr.statusText));
                }
            });

            xhr.addEventListener('error', () => reject(new Error(xhr.statusText)));
            xhr.addEventListener('abort', () => reject(new Error('Request aborted')));

            xhr.send(options.body);
        });

It’s very easy for me.