[PDF] javascript tome xxiii promises - async - await

What is the difference between asynchronous and promise based API?

With that design, you call the asynchronous function, passing in your callback function. The function returns immediately and calls your callback when the operation is finished. With a promise-based API, the asynchronous function starts the operation and returns a Promise object.

What happens if await gets a non-promise object?

If await gets a non-promise object with .then, it calls that method providing the built-in functions resolve and reject as arguments (just as it does for a regular Promise executor). Then await waits until one of them is called (in the example above it happens in the line (*)) and then proceeds with the result.

How to create a promise in JavaScript?

To create a promise we need to use the Promise constructor function like this: The Promise constructor takes a function as an argument and that function internally receives resolve and reject as parameters. The resolve and reject parameters are actually functions that we can call depending on the outcome of the asynchronous operation.

What is a promise in ES6?

A promise represents an asynchronous operation whose result will come in the future. Before ES6, there was no way to wait for something to perform some operation. For example, when we wanted to make an API call, there was no way to wait until the results came back.