why to put [0] on the result of children (':first')?

Please explain the [0] in this code:

$(function (){      
    var $cont = $('#cont');     
    var $el = $cont.children(':first'), el = $el[0];    
}); 

I tried some 'alert' to find the text, but i don't really understand why we precise the index, while we already kno开发者_运维问答w that we're pointing on the "first" children of the div... ?

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In the above example, $el is a jQuery object containing one element which is ensured by the :first selector. On the other hand, el contains the underlying DOM element of the first (and only) item in the $el jQuery object.

You can access native properties of the el variable, like .innerHTML. You can use all jQuery supported operations on $el, like $el.html().

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A jQuery object normally contains a collection/array of DOM nodes. For instance, if you are iterating over a jQuery object such as -

$('div').each(function() {

  //In this context, this refers to the DOM node
  //and $(this) refers to a jQuery object for that particular DOM node
  //therefore $(this)[0] == this

   this.innerHTML = 'foo';
   $(this).html('foo');
   $(this)[0].innerHTML = 'foo';

});

You can also use .get() for a similar effect.

//retrieves an array of native DOM nodes using jQuery
var allDivNodes = $('div').get();

//retrieves the first node retrieved by the selector
var firstDiv = $('div').get(0);

//this is the same as get(0), however, using an array accessor can throw an exception
var firstDiv = $('div')[0];

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All jQuery queries return a list of all objects that matched. :first does not guarantee a single result, thus [0] grabs a single element.

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var $el = $cont.children(':first')

If the selector matches, that gives you an array-like object with the matched element. What you want is the matched element - that's why you use the [0] - to get the first (and only) element of the returned "array".

This is basically like the difference between: [element] and element (Where, [element][0] = element)

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[0] is the same thing as using get(0) to obtain the DOM element, not the jQuery element.