jashkenas/coffeescript

Bug: Wrong `this` in bound methods of nested classes

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#5,222 opened on Jul 18, 2019

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 (1 comment) (5 reactions) (0 assignees)CoffeeScript (16,577 stars) (1,980 forks)batch import
bughelp wanted

Description

While upgrading an application to CS2 we ran into the following issue in an obscure usage of nested classes.

Screenshot 2019-07-18 at 16 33 10

As you can see, the nested bound method thought its this is the parent class. This seems to happen on a "parse" level, because it's replaced in the code. This lead to tricky to catch bug on our side.

Contributor guide

Tech stack
javascript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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.
needs investigation
Prerequisites
CoffeeScript class syntaxJavaScript 'this' bindingbasic understanding of compilation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the CoffeeScript parser, specifically how bound methods in nested classes are compiled. Look at the relevant source files such as 'src/nodes.coffee' or 'src/grammar.coffee' to understand how 'this' is resolved. Check if there are any existing tests for nested class bound methods. Consider reproducing the issue with a minimal example and examining the generated JavaScript output to identify where the binding goes wrong.
Bug: Wrong `this` in bound methods of nested classes · jashkenas/coffeescript#5222 | Good First Issue