Computer Science

How to write Promises in JavaScript

In Computer Science, a promise represents the eventual completion of a operation. In JavaScript, it allows asynchronous execution of functions without blocking the main thread.

Lim Xing Zhi

Jun 14, 2020 — 2 min read

In Computer Science, a promise represents the eventual completion of a operation. In JavaScript, it allows asynchronous execution of functions without blocking the main thread.

I will be using typescript for most of the examples here. Think of it as JavaScript, but with strong types and a transpile stage that catches all my dumb errors before converting to JavaScript.

How does a promise looks like

I am using typescript for a clearer example

Promises requires only one parameter, a function expression that is immediately executed (promiseExecution). This function accepts 2 parameters, first parameter will be called when the promise is fulfilled (resolve), the second will be called when the promise is rejected (reject).

If you look at the logs printed, the function expression (promiseExecuted) is immediately executed when the promise is declared.

Resolving a promise

Referring to the example above, a promise is fulfilled when the resolve function is called. The caller can pass the resolve function with the promiseA.then syntax.

The inverse applies for promiseA.reject. A promise is rejected when the reject function is called. This can be implemented with the promiseA.catch syntax.

Promise Usage

One of the most prominent usage of Promises is the fetch API. The fetch API is a browser RESTful interface that allows browser-side JavaScript to do HTTP based requests.

fetch('http://example.com/movies.json')
  .then(response => response.json())
  .then(data => console.log(data));

fetch API example

What you see in this example is known as promise chaining. The fetch function returns a promise that is resolved when the JSON data is returned from the RESTful endpoint. .next returns a promise. That second promise will be resolved when the response from the first promise is parsed as a JSON object. Then, in the last function, the JSON data is printed to the console.

Async functions

Introduced in ES7, async functions always return a promise. Without the await keyword, the promise will always resolve immediately after execution of its main thread.

With the typescript syntax, we can see that an async function always returns a promise.