handler.ownKeys()
The handler.ownKeys()
method is a trap for
Reflect.ownKeys()
.
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Syntax
const p = new Proxy(target, {
ownKeys: function(target) {
}
});
Parameters
The following parameter is passed to the ownKeys()
method.
this
is bound to the handler.
target
- The target object.
Return value
The ownKeys()
method must return an enumerable object.
Description
The handler.ownKeys()
method is a trap for
Reflect.ownKeys()
.
Interceptions
This trap can intercept these operations:
Invariants
If the following invariants are violated, the proxy will throw a
TypeError
:
- The result of
ownKeys()
must be an array. - The type of each array element is either a
String
or aSymbol
. - The result List must contain the keys of all non-configurable own properties of the target object.
- If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.
Examples
Trapping of getOwnPropertyNames
The following code traps Object.getOwnPropertyNames()
.
const p = new Proxy({}, {
ownKeys: function(target) {
console.log('called');
return ['a', 'b', 'c'];
}
});
console.log(Object.getOwnPropertyNames(p)); // "called"
// [ 'a', 'b', 'c' ]
The following code violates an invariant.
const obj = {};
Object.defineProperty(obj, 'a', {
configurable: false,
enumerable: true,
value: 10 }
);
const p = new Proxy(obj, {
ownKeys: function(target) {
return [123, 12.5, true, false, undefined, null, {}, []];
}
});
console.log(Object.getOwnPropertyNames(p));
// TypeError: proxy [[OwnPropertyKeys]] must return an array
// with only string and symbol elements
Specifications
Browser compatibility
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