SequentialPromises.ts

function methodThatReturnsAPromise ( id ) {
    return new Promise( ( resolve, reject ) => {
        setTimeout( () => {
            console.log( `Processing ${ id }` );
            resolve( id );
        }, 1000 );
    } );
}

let result = [ 1, 2, 3 ].reduce( async ( accumulatorPromise, nextID ) => {
    await accumulatorPromise;
    return methodThatReturnsAPromise( nextID );
}, Promise.resolve() );

result.then( e => {
    console.log( "Resolution is complete! Let's party." )
} );

// let result = [ 1, 2, 3 ].reduce( ( accumulatorPromise, nextID ) => {
//     return accumulatorPromise.then( () => {
//         return methodThatReturnsAPromise( nextID );
//     } );
// }, Promise.resolve() );