zalgo-promise

zalgo-promise

A promise library that does not automatically resolve promises asynchronously, unless you do so manually.

https://medium.com/@bluepnume/intentionally-unleashing-zalgo-with-promises-ab3f63ead2fd

Quick Start

npm install --save zalgo-promise

Global

<script src="zalgo-promise.js"></script>
<script>
    new ZalgoPromise( ... );
</script>

CommonJS

var ZalgoPromise = require('zalgo-promise');

new ZalgoPromise( ... );

ES6

import { ZalgoPromise } from 'zalgo-promise';

new ZalgoPromise( ... );

Rationale

Promises are asynchronous by default. This means that:

Promise.resolve('foo').then(function(result) {
    console.log(result);
});

console.log('bar');

Will log:

bar
foo

Even if we don't do anything explicitly asynchronous in the promise, it becomes asynchronous by default.

This is, generally speaking, a good thing™

However, this is a problem when:

• You're using a browser which doesn't have native Promise support
• You're using a Promise shim, which has to use setTimeout() to guarantee promises are resolved asynchronously
• You need to deal with cases where the browser deprioritizes setTimeout(), for instance, you're in a popup window but still want to communicate with the parent window

In this case, any promises you run in the unfocused window will try to run setTimeout(), the browser will deprioritize those calls, and your code will hang.

ZalgoPromise attempts to resolve this problem by introducing promises which are not asynchronous by default, only if you explicitly do something asynchronous like an ajax call, a post-message, a setTimeout or something else.

var promise = new ZalgoPromise(function(resolve) {
    resolve();
});

promise.then(function() {
    // This function will be called *synchronously*
});

var promise = new ZalgoPromise(function(resolve) {
    setTimeout(resolve, 100);
});

promise.then(function() {
    // This function will be called *asynchronously*
});