How to use 'javascript enumerate' in JavaScript

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(function testProtoProxyEnumerate() {
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  var keys = ['a', 'b', 'c', 'd'];
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  var handler = {
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   enumerate() { return keys[Symbol.iterator]() },
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   has(target, key) { return false }
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  };
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  var proxy = new Proxy({}, handler);
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  var seen_keys = [];
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  for (var i in proxy) {
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    seen_keys.push(i);
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  }
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  assertEquals([], seen_keys);
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  handler.has = function(target, key) { return true };
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  for (var i in proxy) {
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    seen_keys.push(i);
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  }
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  assertEquals(keys, seen_keys);
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  o = {__proto__:proxy};
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  handler.has = function(target, key) { return false };
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  seen_keys = [];
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  for (var i in o) {
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    seen_keys.push(i);
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  }
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  assertEquals([], seen_keys);
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  handler.has = function(target, key) { return true };
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  seen_keys = [];
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  for (var i in o) {
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    seen_keys.push(i);
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  }
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  assertEquals(keys, seen_keys);
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})();