异步forEach函数返回未定义

Question

I'm trying to build an asynchronous forEach function, but unfortunately it returns the value undefined instead of the content of the array for which I had hoped for. What am I doing wrong?

function asyncforEach(array, cb) {
    array.forEach(function(){
        setTimeout(cb, 0);
    });
}

asyncforEach([1,2,3,4], function(i) {
    console.log(i)
});

Answer 1

Pass the variable ( value ) to the callback, in setTimeout .

 function asyncforEach(array, cb) { array.forEach(function(value){ // value: The current array entry setTimeout(cb, 0, value); // Pass it to the callback when setTimeout invokes it. }); } asyncforEach([1,2,3,4], function(i) { console.log(i) }); 

Any parameters passed to setTimeout after the function and delay parameters, are passed to the callback:

var timeoutID = scope.setTimeout(function[, delay, param1, param2, ...]);

Answer 2

You missed to get the item in the forEach function parameter and pass the item into the setTimeout , Also call the cb function with passed array item into it. This will force the cb function to be called inside the arrow function, when the time will be out.

 function asyncforEach(array, cb) { array.forEach(function(i) { //-----------------^--- setTimeout( () => cb(i), 0); //----------^^^^^^^^^^^---- }); } asyncforEach([1,2,3,4], function(i) { console.log(i) });