javascript-promise

Promise Fundamentals

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A promise represents a value that’s unknown now, that may become known in future.

Promise manages a single asynchronous value that can be handled in the future.

Think of it as a ride hailing app. When you request a ride, the driver make a promise to give you ride.

• While you’re waiting, the ride is pending
• If everything goes right, the ride will be fulfilled
• But in some case the ride might get rejected

Whatever may the case, the original request is settled.

As a developer, you may want to create a promise to represent a asynchronous value. But more often than not. you’ll be promises to use result of an asynchronous operation.

When a Promise is constructed it start with pending state. It’s your job to define a callback function called executor , which is a function that resolves a value, or rejects (error). The consumer of the promise consumes it using the then method, which is waiting for the asynchronous value to be fulfilled. When that happens we can call a function in then with async value being it’s argument. To handle rejection use the catch method which again gets a callback having error as it’s argument. You can use finally to execute any code, no matter what. then, catch returns promises , so they can be chained together.

Async Await

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Anything we don on the web is time consuming OR blocking , therefore async await is very important for a JS dev.

General Overview of Event Loop

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Both the brower & node JS are always running a single threaded event loop to run your code.

• On the first go around it run all the sync code but might also queue up async events to be called back later, something like fetching from API, in this case the event loop will on doing it’s thing and fetching for API will happen in separate thread pool. In some point in future the we’ll fetch some data from the API and will let the event loop know that it’s (async event) is ready to be called back.
• Interesting thing is that
  • If it’s a macro task like setTimeout OR setInterval, then it will be executed on the next event loop
  • But if it’s a micro task like a fulfilled promise then it will be called back before the start of the next event loop