How do I execute something after all promises within a loop have resolved?

Example code:
``` javascript

// ... url, headers, parameters for the requests

// Item we are iterating over and we actually use within the requests
var example = {
  'Weekly',
  'Monthly',
  'Yearly'
};

// Set up empty results object
var results = {};

// Loop over filters and create two queries for each filter
Object.keys( example ).forEach( ( filter, index ) => {

    // ... Preparing parameters depending on currently looped item

    // Make first request
    makeRequest( url, headers, 'GET', parameters )
        .then( ( data ) => {

            // ... Doing stuff with data depending on currently looped item
            var firstData = data;

            // Make another request with other parameters
            return makeRequest( endpoint, headers, 'GET', parameters );

        } )
        .then( ( data ) => {

            // Add both things to resultsBuild results array
            results[ filter ] = {
                first: firstData,
                second: data
            };

            if ( index === Object.keys( example ).length - 1 )
                exampleCallbackMethod( results ) // This should only be executed when the for loop is finished and all promises are resolved

        } )
        .catch( ( error ) => {

            console.error( error );

        } );

} );
```