So I figured, “Why not write my own Promise.map?”. Promise.all sets no limit on the number of “threads.” If my list consists of 1,000 items, 1,000 API calls will be fired all at the same time. // If one of the promises in the original array rejects, // the iteration will stop once it reaches it. Since you do not pass in a function with Promise.all, all the pending promises must have been generated somewhere else (by invoking the API call function). So the mapper function will be called three times and it will be called again only after at least one of the Promises resolves. Learn how Promise.map can be implemented in native Promise API without importing bluebird. ", "5.txt!"]. ", "2.txt! JavaScript’s Promise lends itself well to staggering because it is easy to create waterfall behavior from Promise. The problem of staggering API calls for web scraping can be addressed with Promise.map. Data scraping on the web is usually done in two steps: It is certainly a bad idea to wrap all the step 2 API calls in a giant Promise.all. The mapper returns a promise or a thenable, it is awaited before continuing to the next iteration. If concurrency is not set, it defaults to the other extreme, which is Infinity. One by one, you will visit the items on the list from step 1 to fetch out the details. The basic implementation above has two outstanding issues: How can you ensure that a thenable is returned from mapper? Playing with the first example with and without limits, and seeing how it affects the duration when reading 20 files: The order map calls the mapper function on the array elements is not specified, there is no guarantee on the order in which it'll execute the maper on the elements. Bluebird is a fully featured JavaScript promises library with unmatched performance. Invokes the mapper function on the input. It ensures that the .then method is always present and avoids the need for type checking and separate handling. Bluebird is a fully featured JavaScript promises library with unmatched performance. Places the resolved item in the output array (at the corresponding index). Promise.mapSeries is very similar to Promise.each. I added an explicit note in the docs for .map.. npm install bluebird. Promises returned by the mapper function are awaited for and the returned promise doesn't fulfill until all mapped promises have fulfilled as well. Bluebird is a fully featured promise library with focus on innovative features and performance. Take the example of a property portal. Line 18 replaces array indexing with an iterator object. Preferring to use native Promise as much as I can, I find installing a whole bluebird module for a single function uneconomical. With that said, it behaves similarly to Promise.all, invoking mapper upfront on the list from step 1 fetch! 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