I understand the basic structure of a For loop in JavaScript. I was looking at the following example:

function howMany(selectObject) {
  var numberSelected = 0;
  for (var i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

On the Fourth line I don't understand what you would call the [i] in terminology and why it is square brackets?

I understand the basic structure of a For loop in JavaScript. I was looking at the following example:

function howMany(selectObject) {
  var numberSelected = 0;
  for (var i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

On the Fourth line I don't understand what you would call the [i] in terminology and why it is square brackets?

• javascript

• 5 You never saw an array ? – Denys Séguret Commented Nov 10, 2015 at 12:51
• Get value in array at position i – Joakim M Commented Nov 10, 2015 at 12:51
• w3schools./js/js_arrays.asp – Joakim M Commented Nov 10, 2015 at 12:52
• 1 selectObject is an object having array property options. now this array contains objects, having one property selected – ozil Commented Nov 10, 2015 at 12:53
• 2 I know them as indexer's – Johan Commented Nov 10, 2015 at 13:09

4 Answers 4

[] is a way of selecting a property from an object given a specific key, in this case the key (or index) is i and the object is an array. In an array an index can go from 0 to the length of the array - 1.

In an object a key is the name of any property within that object. For example, you can also select the value of the property key selected from the object selectObject.options[i] by using the following: selectedObject.options[i]['selected'].

As an alternative to your for loop, you could use a for in loop. Which works on objects (and arrays).

for (var key in selectObject.options) {
    if (selectObject.options[key].selected) {
        numberSelected++;
    }
} 

the [i] is used to address variables in for example an array.

Lets say you have an array names containing sarah and john. names[0] would return sarah.

What your for loop does is go over all the entries in selectObject.options and looks at the value of selected (most likely a true/false).

selectObject.options returns an array, and the [ ], is the way to get an element from the array, using its index (the i in your case)

Say you had an array of strings like so:

var arr = ["this", "is", "an", "array", "of", "strings"];

and you want to access one of the array's elements, you would:

console.log(arr[5]); // prints "strings" to the console

function howMany(selectObject) {
  var numberSelected = 0;
  for (var i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

In this above code why is numberSelected, and in the coditional statement numberSelected++