const request = indexedDB.open( 'myDB',1 );
var db;

request.onupgradeneeded = ...

request.onsuccess = function() {
    db = this.result; // Average 8ms
};


self.onfetch = function(e)
{
    const requestURL = new URL( e.request.url ),path = requestURL.pathname;

    if( path === '/test' )
    {
        const response = new Promise( function( resolve )
        {
            console.log( performance.now(),typeof db ); // Average 15ms

            db.transaction( 'cache' ).objectStore( 'cache' ).get( 'test' ).onsuccess = function()
            {
                resolve( new Response( this.result,{ headers: { 'content-type':'text/plain' } } ) );
            }
        });

        e.respondWith( response );
    }
}

ServiceWorker启动时可能会失败,如果是这样,在ServiceWorker中访问indexedDB的有效方法是什么？