Asynchronous a story in JavaScript

controlling the timing of operations by the use of pulses sent when the previous operation is completed rather than at regular intervals.

Asynchronous a story in JavaScript

a single-threaded,
non-blocking,
synchronous,
single concurrent,

Asynchronous a story in JavaScript

Asynchronous programming, as we know now in JavaScript, can only be achieved with functions being first-class citizens of the language:
they can be passed around like any other variable to other functions.

Function as first class citizen

Asynchronous a story in JavaScript - not!?

Up until recently (ES6), JS itself has actually never had any direct notion of asynchrony built into it;

Asynchronous a story in JavaScript - Callback

Callbacks are the fundamental unit of asynchrony in JS.

Asynchronous a story in JavaScript - Callback

Callbacks are the fundamental unit of asynchrony in JS. But they’re not enough for the evolving landscape of async programming as JS matures.

Asynchronous a story in JavaScript - Callback

A lose of control ( Inversion of Control ? can i trust you ? )
Hard to reason about ( its a time loooooooop and the temporal dependency)

Medieval era

Asynchronous a story in JavaScript - Promises

exceptions become errors
immutable once resolved
callback hell still here