turbo-combine-reducers/index.js

Speed-optimized drop-in replacement for Redux's combineReducers

function combineReducers( reducers ) {
	var keys = Object.keys( reducers ),
		getNextState;

	getNextState = ( function() {
		var fn, i, key;

		fn = 'return {';
		for ( i = 0; i < keys.length; i++ ) {
			// Rely on Quoted escaping of JSON.stringify with guarantee that
			// each member of Object.keys is a string.
			//
			// "If Type(value) is String, then return the result of calling the
			// abstract operation Quote with argument value. [...] The abstract
			// operation Quote(value) wraps a String value in double quotes and
			// escapes characters within it."
			//
			// https://www.ecma-international.org/ecma-262/5.1/#sec-15.12.3
			key = JSON.stringify( keys[ i ] );

			fn += key + ':r[' + key + '](s[' + key + '],a),';
		}
		fn += '}';

		return new Function( 'r,s,a', fn );
	} )();

	return function combinedReducer( state, action ) {
		var nextState, i, key;

		// Assumed changed if initial state.
		if ( state === undefined ) {
			return getNextState( reducers, {}, action );
		}

		nextState = getNextState( reducers, state, action );

		// Determine whether state has changed.
		i = keys.length;
		while ( i-- ) {
			key = keys[ i ];
			if ( state[ key ] !== nextState[ key ] ) {
				// Return immediately if a changed value is encountered.
				return nextState;
			}
		}

		return state;
	};
}

module.exports = combineReducers;