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

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

Interceptions

This trap can intercept these operations:

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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See also