Getting a handle on JS Promises

chains_of_then.js

function timedPromise ( t, label ) {
console.info( 'making promise for ' + label );
    return ( res,rej ) => { 
        setTimeout( () => {
            console.log( label );
            res( label )
        }, t );
    }
}

function makePromise (fulfil) {
    let p = new Promise( timedPromise( 1000, 'first' ) );
    if( fulfil ) {
        console.log('will resolve');
        p.then( value => { new Promise( timedPromise( 100, `after ${value}[0]` ) ) } );
        p.then( value => { new Promise( timedPromise( 100, `after ${value}[1]` ) ) } );
        return p;
    } else {
        console.log('will reject');
        return Promise.resolve(null);
    }
}

makePromise(true).then( function () { console.log("after true"); return 'after' } ).then( ( val ) => new Promise ( timedPromise( 100, `${val} after true` ) ) );
// makePromise(false).then( function () { console.log("after false"); } );

finally_without_finally.js

function firstAsync ( shouldError ) {
    return new Promise( function ( res, rej ) {
        if( shouldError ) rej( 'error' );
        else res( 'no error' );
    });
}

function activate ( message ) {
    console.log( `activate with ${message}` );
    return new Promise( function ( res, rej ) {
        console.log( `second promise with ${message}` );
    });
}

firstAsync(false)
    .then( () => 'first passed', (ex) => `first rejected with ${ex}` )
    .then( activate )
    .then( undefined, (ex) => { console.error( ex ); } );

firstAsync(true)
    .then( () => 'first passed', (ex) => `first rejected with ${ex}` )
    .then( activate )
    .then( undefined, (ex) => { console.error( ex ); } );