When iterating over values, why does typeof(value) return “string” when value is a number? JavaScript

The reason you’re seeing “string” returned in your first loop is that num refers to the array index, not the value of numarray at that index. Try changing your first loop to alert num instead of typeof num and you’ll see that it spits out 0, 1, and 2, which are the indicies and not the values of your array.

When you use a for in loop, you’re iterating over the properties of an object, which is not exactly equivalent to the for loop in your second example. Arrays in JavaScript are really just objects with sequential numbers as property names. They are treated as strings as far as typeof is concerned.

Edit:

As Matthew points out, you’re not guaranteed to get the items in the array in any particular order when using a for in loop, and partly for that reason, it’s not recommended to iterate through arrays that way.

filip-fku asks when it would be useful to use for in, given this behavior. One example is when the property names themselves have meaning, which is not really the case with array indicies. For example:

var myName = {
  first: 'Jimmy',
  last: 'Cuadra'
};

for (var prop in myName) {
  console.log(prop + ': ' + myName[prop]);
}

// prints:
// first: Jimmy
// last: Cuadra

It’s also worth noting that for in loops will also iterate through properties of the object’s prototype chain. For that reason, this is usually how you’d want to construct a for in loop:

for (var prop in obj) {
  if (obj.hasOwnProperty(prop)) {
    // do something
  }
}

This does a check to see if the property was defined by the object itself and not an object it’s inheriting from through the prototype chain.

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