explain JS single thread

explain-single-thread.js

message = 'aha'
setTimeout(() => {
   message = 'happy';
}, 1)
setTimeout(() => {
   console.log('2nd setTimeout: ' + message)
}, 500)
console.log('start: ' + new Date().toISOString())
x = 0
for(let i = 0; i < 500; i++) {
    for(let j = 0; j < 1000000; j++) {
        x = Math.random()
    }
}
console.log('end: ' + new Date().toISOString())
console.log(message)