The handler.has()
method is a trap for the in
operator.
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Syntax
const p = 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 (ECMA-262) The definition of '[[HasProperty]]' in that specification. |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
has | Chrome Full support 49 | Edge Full support 12 | Firefox Full support 18 | IE No support No | Opera Full support 36 | Safari Full support 10 | WebView Android Full support 49 | Chrome Android Full support 49 | Firefox Android Full support 18 | Opera Android Full support 36 | Safari iOS Full support 10 | Samsung Internet Android Full support 5.0 | nodejs Full support 6.0.0 |
Legend
- Full support
- Full support
- No support
- No support
See also
Proxy
handler
in
operatorReflect.has()