in js/reveal.js [3478:3523]
function sortFragments( fragments ) {
fragments = toArray( fragments );
var ordered = [],
unordered = [],
sorted = [];
// Group ordered and unordered elements
fragments.forEach( function( fragment, i ) {
if( fragment.hasAttribute( 'data-fragment-index' ) ) {
var index = parseInt( fragment.getAttribute( 'data-fragment-index' ), 10 );
if( !ordered[index] ) {
ordered[index] = [];
}
ordered[index].push( fragment );
}
else {
unordered.push( [ fragment ] );
}
} );
// Append fragments without explicit indices in their
// DOM order
ordered = ordered.concat( unordered );
// Manually count the index up per group to ensure there
// are no gaps
var index = 0;
// Push all fragments in their sorted order to an array,
// this flattens the groups
ordered.forEach( function( group ) {
group.forEach( function( fragment ) {
sorted.push( fragment );
fragment.setAttribute( 'data-fragment-index', index );
} );
index ++;
} );
return sorted;
}