rtfeldman/seamless-immutable

Incorrectly identifying promises

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#208 opened on Mar 18, 2017

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 (6 comments) (0 reactions) (0 assignees)JavaScript (5,367 stars) (221 forks)batch import
bughelp wanted

Description

I've encountered an issue in the latest (7.1.0) release relating to commit 492d53e705acb6c

The issue is that isPromise makes a huge assumption that any object with a then method is a Promise. If the object is not a Promise, then will be called which may not be desirable.

This can be demonstrated very simply

Immutable({then:function() { console.log("should not be called") }});

I encountered this when using superagent which provides a thenable interface but is not technically a Promise. I suspect there may well be other libraries with a similar approach.

Contributor guide

Tech stack
javascript
Domain
data
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaScript basicsPromise/A+ thenable concept
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Examine the `isPromise` function in the codebase, likely in a utility file. The issue references commit 492d53e705acb6c where the assumption was introduced. Review the comments for suggested fixes, such as checking that `then` is a function before calling it. Consider using `Promise.resolve(value)` to detect thenables more safely, but be aware of side effects.