In the documentation of the Proxy ownKeys
trap on MDN it states that it will intercept Object.keys()
calls:
This trap can intercept these operations:
Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Object.keys()
Reflect.ownKeys()
However, from my tests it doesn't seem to work with Object.keys
:
const proxy = new Proxy({}, {
ownKeys() {
console.log("called")
return ["a", "b", "c"]
}
})
console.log(Object.keys(proxy))
console.log(Object.getOwnPropertyNames(proxy))
console.log(Reflect.ownKeys(proxy))
In the documentation of the Proxy ownKeys
trap on MDN it states that it will intercept Object.keys()
calls:
This trap can intercept these operations:
Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Object.keys()
Reflect.ownKeys()
However, from my tests it doesn't seem to work with Object.keys
:
const proxy = new Proxy({}, {
ownKeys() {
console.log("called")
return ["a", "b", "c"]
}
})
console.log(Object.keys(proxy))
console.log(Object.getOwnPropertyNames(proxy))
console.log(Reflect.ownKeys(proxy))
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edited May 2, 2023 at 15:57
sdgluck
asked Dec 17, 2020 at 11:38
sdglucksdgluck
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3 Answers
Reset to default 9The reason is simple: Object.keys
returns only properties with the enumerable flag. To check for it, it calls the internal method [[GetOwnProperty]]
for every property to get its descriptor. And here, as there’s no property, its descriptor is empty, no enumerable flag, so it’s skipped.
For Object.keys
to return a property, we need it to either exist in the object, with the enumerable flag, or we can intercept calls to [[GetOwnProperty]] (the trap getOwnPropertyDescriptor does it), and return a descriptor with enumerable: true.
Here’s an example of that:
let user = { };
user = new Proxy(user, {
ownKeys(target) { // called once to get a list of properties
return ['a', 'b', 'c'];
},
getOwnPropertyDescriptor(target, prop) { // called for every property
return {
enumerable: true,
configurable: true
/* ...other flags, probable "value:..." */
};
}
});
console.log( Object.keys(user) ); // ['a', 'b', 'c']
Source
Object.keys
returns only the enumerable own properties of an object. Your proxy doesn't have such, or at least it doesn't report them in its getOwnPropertyDescriptor
trap. It works with
const proxy = new Proxy({}, {
ownKeys() {
console.log("called ownKeys")
return ["a", "b", "c"]
},
getOwnPropertyDescriptor(target, prop) {
console.log(`called getOwnPropertyDescriptor(${prop})`);
return { configurable: true, enumerable: true };
}
})
console.log(Object.keys(proxy))
console.log(Object.getOwnPropertyNames(proxy))
console.log(Reflect.ownKeys(proxy))
const proxy = new Proxy({}, {
ownKeys: function () {
console.log("called")
return ["a", "b", "c"]
}
});