Question

var test = $();
test.push( $('div')[0] );

This works. Any idea why?
Since push is a array method, I don’t understand why this works in this case, any idea?

Is there a practical use for it since you can use .add() ?

Answer 1

They borrow .push() from Array.prototype.

https://github.com/jquery/jquery/blob/1.6.2/src/core.js#L70

and

https://github.com/jquery/jquery/blob/1.6.2/src/core.js#L296

Example: http://jsfiddle.net/HgS2R/1/

var MyObj = function(){};
MyObj.prototype.push = Array.prototype.push;

var inst = new MyObj;

inst.push( "test" );

document.write( inst[ 0 ] + '<br>' + inst.length );  // test, 1

Just be aware that instances of your custom constructor won’t behave identically to an actual Array.

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EDIT:

With respect to the edit in your question, .push() is not identical to .add().

The .push() method is for adding a DOM element to an existing jQuery object.

The .add() method can accept a DOM element, or another jQuery object with 1 or more DOM elements, and it returns a new jQuery object with the new elements added.

It’s more like a .concat().