JavaScript / jQuery Methods

You should note that jQuery methods are for applying to jQuery collections rather than to native arrays / objects.

.forEach()

.forEach(el, index) is not a jQuery method . It is rather a native method of JavaScript introduced in version ES5. The jQuery match for this method is .each(index, el) that has the callback arguments reversed, and passes el to it context to the callback.

This method (both the native and jQuery version) is iterators. They serve to traverse an array or jQuery collection.

.slice()

This method exists in jQuery and also in native JavaScript. Again they are similar and do the same thing, respectively in jQuery collections or native arrays.

The method .slice() itself serves to create copies of pieces of an array / collection.

What does .call() or [].slice.call() ?

The methods I described above (and many others too) have an internal method that is .call() . This method lets you call slice , forEach or other passing as context the first argument that this method receives. Do you remember that I mentioned that .forEach of jQuery is called .each and passes the collection element as context? I want to use jQuery to call the%

The idea here is to be able to call a function or method by controlling the context in which it will be run.

Using this is basically the same as .each.call(el, etc... , but shorter :) And allows you to convert Lists into arrays. Lists are not arrays, but can be converted into arrays. An example of a List is the result of [].slice.call , which does not have a Array.prototype.slice.call method but can be iterated if used like this:

var divs = [].slice.call(document.querySelectorAll('div'));
// e depois:
divs.forEach(function(el){
    // fazer algo sobre o elemento <div> a ser iterado
});