Accessing outside variables inside the 'then' function

Question

I am new to nodejs. Using bluebird promises to get the response of an array of HTTP API calls, and storing derived results in an ElasticSearch.

Everything is working fine, except I am unable to access the variables within the 'then' function. Below is my code:

Promise.map(bucket_paths, function(path) {
    this.path = path;
    return getJson.getStoreJson(things,path.path);
}, {concurrency:1}).then(function(bucketStats){
    bucketStats.map(function(bucketStat) {   
        var bucket_stats_json = {};
        bucket_stats_json.timestamp = new Date();
        bucket_stats_json.name = path.name ==> NOT WORKING
    });
});

How can I access the path.name variable within the 'then' ? Error says 'path' is undefined.

Answer 1

The best way to do this is to package the data you need from one part of the promise chain into the resolved value that is sent onto the next part of the chain. In your case with Promise.map(), you're sending an array of data onto the .then() handler so the cleanest way to pass each path down to the next stage is to make it part of each array entry that Promise.map() is resolving. It appears you can just add it to the bucketStat data structure with an extra .then() as show below. When you get the data that corresponds to a path, you then add the path into that data structure so later on when you're walking through all the results, you have the .path property for each object.

You don't show any actual result here so I don't know what you're ultimately trying to end up with, but hopefully you can get the general idea from this.

Also, I switched to Promise.mapSeries() since that's a shortcut when you want concurrency set to 1.

Promise.mapSeries(bucket_paths, function(path) {
    return getJson.getStoreJson(things,path.path).then(bucketStat => {
        // add the path into this item's data so we can get to it later
        bucketStat.path = path;
        return bucketStat;
    });
}).then(function(bucketStats){
    return bucketStats.map(function(bucketStat) {   
        var bucket_stats_json = {};
        bucket_stats_json.timestamp = new Date();
        bucket_stats_json.name = bucketStat.path.name;
        return bucket_status_json;
    });
});