Refactoring�Function Name Inference:

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�For "consensus awaiting tests"

Author: André Bargull

Presenter: Daniel Ehrenberg

January 2019 TC39

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Class name availability in static fields

• Class name is set after static initializers run!

class C {� static foo = this.name;�}�print(C.name === "C");�print(C.foo === "");

Problem: Class names and a @frozen decorator

• Decorators run before name is added, so it's already frozen!

function frozen(desc) {� desc.elements.push({� kind: "hook",� finish: Object.freeze� });�}��@frozen�class B { } // TypeError!

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Problem: FNI over decorated fields

• Unclear how to permit function name inference to work on decorated field initializers

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const id = desc => desc;

class D {� @id fn = () => {};�}

const d = new D;�d.fn.name // ""

Opportunity: Enumeration order web reality

$ eshost -e "Reflect.ownKeys(class C { static method() { } })"

#### V8

length,prototype,method,name

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#### JSC

length,name,prototype,method

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#### ChakraCore

prototype,name,method,length

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#### SpiderMonkey

prototype,method,length,name

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anba's solution: NamedEvaluation

https://github.com/tc39/ecma262/pull/1372

• Trick: FNI names precede the thing they are naming
• NamedEvaluation abstract op passes name into evaluation
• Class name assignment moves to ClassDefinitionEvaluation
• Observable change: enumeration order of class properties
• Shipping in Chrome 72

Consensus, awaiting tests?