ractive/src/shared/summariseSpliceOperation.js

Next-generation DOM manipulation

// This function summarises a splice operation, as returned from
// getSpliceEquivalent()

export default function ( array, args ) {
	var rangeStart, rangeEnd, newLength, addedItems, removedItems, balance;

	if ( !args ) {
		return null;
	}

	// figure out where the changes started...
	rangeStart = +( args[0] < 0 ? array.length + args[0] : args[0] );

	// ...and how many items were added to or removed from the array
	addedItems = Math.max( 0, args.length - 2 );
	removedItems = ( args[1] !== undefined ? args[1] : array.length - rangeStart );

	// It's possible to do e.g. [ 1, 2, 3 ].splice( 2, 2 ) - i.e. the second argument
	// means removing more items from the end of the array than there are. In these
	// cases we need to curb JavaScript's enthusiasm or we'll get out of sync
	removedItems = Math.min( removedItems, array.length - rangeStart );

	balance = addedItems - removedItems;
	newLength = array.length + balance;

	// We need to find the end of the range affected by the splice
	if ( !balance ) {
		rangeEnd = rangeStart + addedItems;
	} else {
		rangeEnd = Math.max( array.length, newLength );
	}

	return {
		rangeStart: rangeStart,
		rangeEnd: rangeEnd,
		balance: balance,
		added: addedItems,
		removed: removedItems
	};
}