node.js - Parallel operations with Promise.all?

Question

I'm led to believe that Promise.all executes all the functions you pass it in parallel and doesn't care what order the returned promises finish.

But when I write this test code:

    function Promise1(){
        return new Promise(function(resolve, reject){
            for(let i = 0; i < 10; i++){
                console.log("Done Err!");
            }
            resolve(true)
        })
    }
    
    function Promise2(){
        return new Promise(function(resolve, reject){
            for(let i = 0; i < 10; i++){
                console.log("Done True!");
            }
            resolve(true)
        })
    }
    
    Promise.all([ 
        Promise1(),
        Promise2()
    ])
    .then(function(){
        console.log("All Done!")
    })

Answer 1

It's because your Promises are blocking and synchronous! Try something with a timeout instead of a synchronous loop:

    function randomResolve(name) {
      return new Promise(resolve => setTimeout(() => {
        console.log(name);
        resolve();
      }, 100 * Math.random()));
    }
    
    Promise.all([ 
        randomResolve(1),
        randomResolve(2),
        randomResolve(3),
        randomResolve(4),
    ])
    .then(function(){
        console.log("All Done!")
    })