gistfile1.js

function promiseReduce( entries, reducer, initialValue ) {
    return new Promise(function( resolve, reject ) {
        (function next( i, memo ) {
            Promise.resolve( reducer( memo, entries[ i ], i, entries ) ).then(function( returnValue ) {
                if ( i === entries.length - 1 ) {
                    resolve( returnValue );
                } else {
                    next( i + 1, returnValue );
                }
            }).catch( reject )
        }( 0, initialValue ));
    });
}