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module: warn on using unfinished circular dependency #29935

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wants to merge 3 commits into from
Closed

module: warn on using unfinished circular dependency #29935

wants to merge 3 commits into from

Commits on Oct 11, 2019

  1. module: warn on using unfinished circular dependency 

    Warn when a non-existent property of an unfinished module.exports
object is being accessed, as that very often indicates the presence
of a hard-to-detect and hard-to-debug problem.

This mechanism is only used if `module.exports` is still a
regular object at the point at which the second, circular `require()`
happens.

The downside is that, temporarily, `module.exports` will have a
prototype other than `Object.prototype`, and that there may
be valid uses of accessing non-existent properties of unfinished
`module.exports` objects.

Performance of circular require calls in general is not
noticeably impacted.

                                               confidence improvement accuracy (*)   (**)  (***)
     module/module-loader-circular.js n=10000                 3.96 %       ±5.12% ±6.82% ±8.89%

Example:

    $ cat a.js
    'use strict';
    const b = require('./b.js');

    exports.fn = () => {};
    $ cat b.js
    'use strict';
    const a = require('./a.js');

    a.fn();
    $ node a.js
    (node:1617) Warning: Accessing non-existent property 'fn' of module exports inside circular dependency
    /tmp/b.js:4
    a.fn();
      ^

    TypeError: a.fn is not a function
        at Object.<anonymous> (/tmp/b.js:4:3)
        [...]
    @addaleax
    addaleax committed Oct 11, 2019
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  2. fixup! module: warn on using unfinished circular dependency

    @addaleax
    addaleax committed Oct 11, 2019
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  3. fixup! module: warn on using unfinished circular dependency

    @addaleax
    addaleax committed Oct 11, 2019
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