handler.has()
The handler.has()
method is a trap for the
in
operator.
Try it
Syntax
new Proxy(target, {
has: function(target, prop) {
}
});
Parameters
The following parameters are passed to has()
method. this
is
bound to the handler.
target
-
The target object.
prop
-
The name or
Symbol
of the property to check for existence.
Return value
The has()
method must return a boolean value.
Description
The handler.has()
method is a trap for the
in
operator.
Interceptions
This trap can intercept these operations:
- Property query:
foo in proxy
- Inherited property query:
foo in Object.create(proxy)
with
check:with(proxy) { (foo); }
Reflect.has()
Invariants
If the following invariants are violated, the proxy will throw a
TypeError
:
- 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.
const p = new Proxy({}, {
has: function(target, prop) {
console.log('called: ' + prop);
return true;
}
});
console.log('a' in p); // "called: a"
// true
The following code violates an invariant.
const obj = { a: 10 };
Object.preventExtensions(obj);
const p = new Proxy(obj, {
has: function(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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See also
Proxy
handler
in
operatorReflect.has()