promise-array-chain.js

var a = {
	foo: [ 'foo1', 'foo2' ],
	bar: [ 'bar1', 'bar2' ],
}

Object.keys( a ).map( (list) => a[list] )
				.reduce( (accum, current) => {
					return accum.then( () => {

						var promises = current.map( (url) => load(url) )
						return Promise.all(promises)
					})
				}, Promise.resolve() )

function load(url) {
	console.log(`loading url ${url}`)
	return new Promise( (resolve, reject) => {

		setTimeout(() => { resolve(url) }, 1000) 
	})
}