src/world.js
function (dt) {
            _.each(
                systems,
                function (system) {
                    if (_.has(system, 'tickStart')) {
                        system.tickStart(dt);
                    }
                }
            );
        }
function (dt) {
            _.each(
                systems,
                function (system) {
                    if (_.has(system, 'tickStart')) {
                        system.tickStart(dt);
                    }
                }
            );
        }
function ( dt ) { _ . each ( systems , function ( system ) { if ( _ . has ( system , 'tickStart' ) ) { system . tickStart ( dt ) ; } } ) ; }
Call all registered system's tick start function.
Call all registered system s tick start function .