Why it is not possible to call forEach on a nodeList?

This is a fundamental thing in JavaScript: you can take a function from one object and apply to any other object. That is: call it with this set to the object you apply the function to. It is possible, because in JavaScript all property names etc. are (plainly speaking) identified by name. So despite NodeList.length being something different then Array.length the function Array.forEach can be applied to anything that exposes property length (and other stuff that forEach requires).

So what happens in your case is that:

• querySelectorAll() returns an object of type NodeList, which happens to expose length property and is enumerable (let's say it is accessible by [] operator); NodeList does not expose forEach function (as you can see i.e here: https://developer.mozilla.org/en-US/docs/Web/API/NodeList) - that's why it's impossible to call forEach directly on the results of querySelectorAll()
• [].forEach returns a function - this a not so clever shortcut for Array.prototype.forEach
• with [].forEach.call(array, …) this function is applied onto an object referenced by array, an object of type NodeList (that is forEach is invoked with array as this in function body, so when inside forEach there is this.length it refers to length in array despite array being NodeList and not real Array)
• this works, because forEach is using properties that Array and NodeList have in common; it would fail if, i.e. forEach wanted to use some property that Array has, but NodeList has not

the NodeList object doesnt contain the method forEach, its a method of the Array object. the below code:

[].forEach.call(array, (function (item) {
    console.log(item);
})); 

is using the forEach method from array and passing it a NodeList.

Another option you have, and arguabiliy better, is to convert your NodeList into an array, like this:

var myArrayOfNodes = [].slice.call(NodeList);

This uses the Array objects slice method to create an array of nodes from a NodeList. This is a better aproach as you can then use an array rather then hacking an array-like object