queue_example.js

const task1 = done => setTimeout(_ => {console.log('1'); done()}, 100)
const task2 = done => setTimeout(_ => {console.log('2'); done()}, 50)
const task3 = done => setTimeout(_ => {console.log('3'); done()}, 25)

const taskRunner1 = queue(1)

taskRunner1.push(task1)
taskRunner1.push(task2)
taskRunner1.push(task3)
// 1
// 2
// 3

const taskRunner2 = queue(2)

taskRunner2.push(task1)
taskRunner2.push(task2)
taskRunner2.push(task3)
// 2
// 3
// 1