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Derived constructors should not be allowed to return primitives #13571

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Derived constructors should not be allowed to return primitives #13571

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513eaa4
Fix Derived constructors should not be allowed to return primitive va…
dhrubesh Jul 19, 2021
6f975ee
fixes indentation and index.js -> input.js
dhrubesh Jul 19, 2021
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9 changes: 7 additions & 2 deletions packages/babel-helpers/src/helpers.js
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Expand Up @@ -615,9 +615,14 @@ helpers.possibleConstructorReturn = helper("7.0.0-beta.0")`
import assertThisInitialized from "assertThisInitialized";

export default function _possibleConstructorReturn(self, call) {
if (call && (typeof call === "object" || typeof call === "function")) {
return call;
if (call) {
if (typeof call === "object" || typeof call === "function") {
return call;
} else {
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This logic doesn't work if the constructor returns, for example, 0 or null (can you add a test for a falsy value?).

It should be something like

if (call && (typeof call === "object" || typeof call === "function")) {
  return call;
} else if (call !== void 0) {
  throw new TypeError("...");
}

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Or

if (call !== void 0) {
  if (typeof call === "object" || typeof call === "function") {
    return call;
  } else {
    // throw errors
  }
}

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My version is 3 bytes shorter when using terser 😛

function f(o){if(o&&("object"==typeof o||"function"==typeof o))return o;if(void 0!==o)throw new TypeError("...")}
function g(o){if(void 0!==o){if("object"==typeof o||"function"==typeof o)return o;throw new TypeError("...")}}

Also, I think yours incorrectly allows returning null.

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but use of 'typeof' is a bad idea and kill perf.

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What do you suggest to use instead?

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@KFlash KFlash Jul 22, 2021 •
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Well. 'typeof obj === 'object';' is almost ~50% slower than other 'typeof'. And 'typeof number' return NaN even if it's not an number - in some cases.

There exist 2 popular utility libraries that has an optimized check for this.

Personally I would have used some kind of meta data and bitwise masks

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There exist 2 popular utility libraries that has an optimized check for this.

I only found is-object (which still uses typeof === "object") and lodash.isObject (which also uses typeof and checks if it's "object"). Why would typeof === "object" be slower than the other types?

Personally I would have used some kind of meta data and bitwise masks

o can be any possible JS value present in the JS code of our users, we cannot use bitwise mask

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I forgot the history behind this, but AngularJS had some good checks in their earlier versions. 1x of their library. I think they checked against the constructor. Also Google had a library a few years back - forgot the name - with fast checks.

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This 'else' is not needed. You already return in the line above.

throw new TypeError("Invalid attempt to return primitive value");
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Thoughts on matching v8's error?

Suggested change
throw new TypeError("Invalid attempt to return primitive value");
throw new TypeError("Derived constructors may only return object or undefined");

}
}

return assertThisInitialized(self);
}
`;
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10 changes: 10 additions & 0 deletions ...ugin-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/exec.js
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@@ -0,0 +1,10 @@
class Bar {}

class Foo extends Bar {
constructor() {
super();
return 3;
}
}

expect(() => new Foo()).toThrow("Invalid attempt to return primitive value");
6 changes: 6 additions & 0 deletions ...gin-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/input.js
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@@ -0,0 +1,6 @@
class Foo extends Bar {
constructor() {
super();
return 3;
}
}
17 changes: 17 additions & 0 deletions ...in-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/output.js
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@@ -0,0 +1,17 @@
var Foo = /*#__PURE__*/function (_Bar) {
"use strict";

babelHelpers.inherits(Foo, _Bar);

var _super = babelHelpers.createSuper(Foo);

function Foo() {
var _this;

babelHelpers.classCallCheck(this, Foo);
_this = _super.call(this);
return babelHelpers.possibleConstructorReturn(_this, 3);
}

return Foo;
}(Bar);