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stream: fix Writable subclass instanceof checks #9088

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stream: fix Writable subclass instanceof checks #9088

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stream: fix `Writable` subclass instanceof checks
addaleax Oct 13, 2016
ba936d8
[squash] fix typo
addaleax Oct 13, 2016
d2ba0ed
[squash] update and move comment
addaleax Oct 14, 2016
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stream: fix Writable subclass instanceof checks 
2a4b068 introduced a regression in where checking
`instanceof` would fail for `Writable` subclasses inside the
subclass constructor, i.e. before `Writable()` was called.

Also, calling `null instanceof Writable` or
`undefined instanceof Writable` would fail due to accessing the
`_writableState` property of the target object.

This fixes these problems.

Ref: #8834 (comment)
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@addaleax
addaleax committed Oct 13, 2016
commit 7fe86726910b18914358ef5a638fed5448ac3274
5 changes: 4 additions & 1 deletion lib/_stream_writable.js
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Expand Up @@ -141,9 +141,12 @@ if (typeof Symbol === 'function' && Symbol.hasInstance) {
realHasInstance = Function.prototype[Symbol.hasInstance];
Object.defineProperty(Writable, Symbol.hasInstance, {
value: function(object) {
if (realHasInstance.call(this, object))
return true;

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@mcollina mcollina Oct 14, 2016

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how does this affect LazyTransform?

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@addaleax addaleax Oct 14, 2016

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@mcollina It works fine (otherwise there are failing tests). LazyTransform is only affected by the conditional in the Writable constructor, and that’s left unchanged here,

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@mcollina mcollina Oct 14, 2016

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can you update the comment below and maybe move it to the constructor then.

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@addaleax addaleax Oct 14, 2016

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@mcollina done, ptal

// Trying to move the `realHasInstance` from the Writable constructor
// to here will break the Node.js LazyTransform implementation.
return object._writableState instanceof WritableState;
return object && object._writableState instanceof WritableState;
}
});
} else {
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16 changes: 16 additions & 0 deletions test/parallel/test-stream-inheritance.js
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Expand Up @@ -27,3 +27,19 @@ assert.ok(!(readable instanceof Transform));
assert.ok(!(writable instanceof Transform));
assert.ok(!(duplex instanceof Transform));
assert.ok(transform instanceof Transform);

assert.ok(!(null instanceof Writable));
assert.ok(!(undefined instanceof Writable));

// Simple inheritance check for `Writable` works fine in a subclass constructor.
function CustomWritable() {
assert.ok(this instanceof Writable, 'inhertis from Writable');
assert.ok(this instanceof CustomWritable, 'inherits from CustomWritable');
}

Object.setPrototypeOf(CustomWritable, Writable);
Object.setPrototypeOf(CustomWritable.prototype, Writable.prototype);

new CustomWritable();

assert.throws(CustomWritable, /AssertionError: inhertis from Writable/);