handler.enumerate()

The handler.enumerate() method used to be a trap for for...in statements, but has been removed from the ECMAScript standard in ES2016 and is deprecated in browsers.

Syntax

var p = new Proxy(target, {
  enumerate(target) {
  }
});

Parameters

The following parameter is passed to the enumerate method. this is bound to the handler.

target

The target object.

Return value

An iterator object.

Description

The handler.enumerate method is a trap for for...in statements.

Interceptions

This trap can intercept these operations:

• Property enumeration / for...in: for (var name in proxy) {...}
• Reflect.enumerate()

Invariants

If the following invariants are violated, the proxy will throw a TypeError:

• The enumerate method must return an object.

Examples

The following code traps for...in statements.

var p = new Proxy({}, {
  enumerate(target) {
    console.log('called');
    return ['a', 'b', 'c'][Symbol.iterator]();
  }
});
for (var x in p) { // "called"
  console.log(x);  // "a"
}                  // "b"
                   // "c"

The following code violates the invariant.

var p = new Proxy({}, {
  enumerate(target) {
    return 1;
  }
});
for (var x in p) {} // TypeError is thrown

Note: Both examples make use of the shorthand syntax for method definitions.

Specifications

Browser compatibility

[1] This feature was supported from version 37 to version 46, but has been removed to comply with the latest ECMAScript standard, see bug 1246318.

See also

• Proxy
• handler
• for...in statements
• Reflect.enumerate()