sinonjs/sinon

Spy's thisValue not pointing to instance when spying constructor

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#1,683 opened on Feb 9, 2018

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BugDifficulty: MediumHelp wantedRegressionhacktoberfestpinned

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Description

This snippet works fine with Sinon 2.0.0 but fails with Sinon 4.2.2:

it('returns correct thisValue', function() {
    window.SomeClass = function() {
        this.counter = 0;
        this.inc = function() {
            this.counter++;
        };
        this.getCounter = function() {
            return this.counter;
        };
    };

    var spy = sinon.spy(window, 'SomeClass');
    var some = new window.SomeClass();
    some.inc();

    expect(spy.thisValues[0].counter).toEqual(1);
    expect(spy.thisValues[0].getCounter()).toEqual(1);
    some.inc();
    expect(spy.thisValues[0].counter).toEqual(2);
    expect(spy.thisValues[0].getCounter()).toEqual(2);
});

It looks like the thisValues[0] object is a proxy and does not refer to the actual instance of some.

This is the error with Sinon 4.2.2:

Expected undefined to equal 1
TypeError: undefined is not a function (evaluating 'spy.thisValues[0].getCounter()')

Contributor guide

Research direction
Investigate the spy's thisValue behavior when spying constructors. Trace the code path where thisValues are recorded and verify that the correct this context is captured.
Tech stack
javascript
Domain
testing
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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
JavaScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65