handler.has()
The handler.has()
method is a trap for the [[HasProperty]]
object internal method, which is used by operations such as the in
operator.
Try it
Syntax
js
new Proxy(target, {
has(target, prop) {
}
});
Parameters
Return value
The has()
method must return a boolean value.
Description
Interceptions
This trap can intercept these operations:
- The
in
operator:foo in proxy
with
check:with(proxy) { (foo); }
Reflect.has()
Or any other operation that invokes the [[HasProperty]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
- A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
- A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
Examples
Trapping the in operator
The following code traps the in
operator.
js
const p = new Proxy(
{},
{
has(target, prop) {
console.log(`called: ${prop}`);
return true;
},
},
);
console.log("a" in p);
// "called: a"
// true
The following code violates an invariant.
js
const obj = { a: 10 };
Object.preventExtensions(obj);
const p = new Proxy(obj, {
has(target, prop) {
return false;
},
});
"a" in p; // TypeError is thrown
Specifications
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-hasproperty-p |
Browser compatibility
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