Rejected state

As explained in Secrets of the JavaScript Ninja, Second Edition

The promise state indicating unsuccessful completion with a rejection reason.

A promise enters the rejected state when an asynchronous task fails: the promised value was not obtained, but the reason for failure is known. section 6.3.2

A promise becomes rejected when:

• Its reject function is called. section 6.3.2
• An unhandled exception occurs during promise handling. section 6.3.2
• It reaches this state, it cannot switch to fulfilled or another state. section 6.3.2

Choosing the failure callback

In the simple Hattori promise, calling reject(...) instead of resolve(...) sends handling to the second callback passed to then, the failure callback. section 6.3

Figure 6.10. States of a promise

Use the promise-state diagram to compare rejection with pending and fulfilled states. section 6.3.2

How Secrets of the JavaScript Ninja, Second Edition covers it

How deep it goes

Defines rejection, explains its triggers, and notes that the state cannot change once reached.

Examples

Uses a Hattori promise with success and failure callbacks, plus an alternate reject call.

What you can do after

Understand when a promise rejects and how its failure callback differs from its success callback.

How it connects

Connects rejection to Promise, pending and fulfilled states, the reject function, and unhandled exceptions.

liveBookmanning.com

Read Secrets of the JavaScript Ninja, Second Edition on liveBook Get the book at manning.com

Related concepts

• promisepart of · A JavaScript Promise is an object representing the eventual completion or failure of an asynchronous operation and its resulting value or reason.
• exceptionproduced by · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
• Promise#catchaddresses · Promise#catch is a JavaScript Promise method that registers a handler for rejection and returns a Promise for the handler's outcome.

This explainer was generated by Manning's knowledge graph from the text of Secrets of the JavaScript Ninja, Second Edition. The section links point to the parts of the book it draws on.