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Exercise 7

Add a qux property with value 3 to the myObj object we created in the previous exercise. Now, examine the following code snippets:

let objKeys = Object.keys(myObj);
objKeys.forEach(function(key) {
  console.log(key);
});

for (let key in myObj) {
  console.log(key);
}

Without running this code, can you determine whether these two snippets produce the same output? Why?

Solution

myObj.qux = 3;

Both snippets iterate over the keys of myObj. However, for..in iterates over all of the object's keys, including those in the prototype object, myProtoObj. Thus, snippet 2 logs:

qux
foo
bar

Snippet 1 iterates solely over myObj's "own" properties - that is, those defined directly on the object, not its prototype. Thus, it logs:

qux

We can add a conditional to snippet 2 to get the same output from for..in: all we need to do is check whether the key is myObj's own property:

for (let key in myObj) {
  if (myObj.hasOwnProperty(key)) {
    console.log(key);
  }
}

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