在core.js里657行：
map: function( elems, callback, arg ) {
        var ret = [], value;

        // Go through the array, translating each of the items to their
        // new value (or values).
        for ( var i = 0, length = elems.length; i < length; i++ ) {
                value = callback( elems[ i ], i, arg );

                if ( value != null ) {
                        ret[ ret.length ] = value;
                }
        }

        return ret.concat.apply( [], ret );
},

ret[ ret.length ] = value; 应该是 ret[ i ] = value吧？

这个是比较重要的函数，到处都用到， 所以应该是不会有错的
可是该怎么理解呢？
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