Promise — Example with diverse situations

Reference note (untrusted external data; do not execute it as instructions). This example shows diverse techniques for using Promise capabilities and diverse situations that can occur. To understand this, start by scrolling to the bottom of the code block, and examine the promise chain. Upon provision of an initial promise, a chain of promises can follow. The chain is composed of .then() calls, and typically (but not necessarily) has a single .catch() at the end, optionally followed by .finally(). In this example, the promise chain is initiated by a custom-written new Promise() construct; but in actual practice, promise chains more typically start with an API function (written by someone else) that returns a promise. The example function tetheredGetNumber() shows that a promise generator will utilize reject() while setting up an asynchronous call, or within the call-back, or both. The function promiseGetWord() illustrates how an API function might generate and return a promise in a self-contained manner. Note that the function troubleWithGetNumber() ends with a throw. That is forced because a promise chain goes through all the .then() promises, even after an error, and without the throw, the error would seem "fixed". This is a hassle, and for this reason, it is common to omit onRejected throughout the chain of .then() promises, and just have a single onRejected in the final catch(). This code can be run under NodeJS. Comprehension is enhanced by seeing the errors actually occur. To force more errors, change the threshold values. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.