This code will output undefined as output. The delete operator is used to delete a property from an object. Here, x is an object which has foo as a property. From the self-invoking function, we are deleting the foo property of object x. After deletion, we are trying to reference the deleted property foo, which results in undefined.

Source

• https://github.com/ganqqwerty/123-Essential-JavaScript-Interview-Question/blob/master/README.md

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var x = { foo : 1};

var output = (function() {

  delete x.foo;

  return x.foo;

})();

​

console.log(output);