寻求帮助了解For循环和.forEach之间的区别

Question

I have two functions that should produce the same result. I would like clarification as to why they do not. I'm not grasping the significant difference that must exist between for-loop and.forEach.

I'd like to though:)

//
//DEMO OBJECT WITH TWO ARRAYS
//
var myArray = {
    myList1 : [1,2,3,4],
    myList2 : [5,6,7,8]
}

//
//FUNCTION ONE: Produces the sum of myArray.myList1 as expected.
//
    var firstWay = function(){
        var sum = 0;

        for(var i = 0; i < myArray.myList2.length; i++){
            sum += myArray.myList2[i];
        }
        return sum
    };

var results2 = firstWay();
console.log(results2);



//FUNCTION TWO PRODUCES NaN
//1.) I don't know why it produced NaN.
//2.) I don't understand why this function wouldn't produce the same result as firstWay.

    var secondWay = function() {
       var sum = 0;
       myArray.myList1.forEach(function(cur){
          sum += cur.value; 
       });
       return sum;
    };

var results1 = secondWay();
console.log(results1);

Answer 1

Remove value from sum += cur.value so it's just sum += cur . The value you need is already contained in cur .

var secondWay = function() {
   var sum = 0;
   myArray.myList1.forEach(function(cur){
      sum += cur; 
   });
   return sum;
};

var results1 = secondWay();
console.log(results1);

Answer 2

Many answers just stand in the doc and might be found by googling.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

callback
Function to execute on each element. It accepts between one and three arguments:

currentValue
The current element being processed in the array
index Optional
The index currentValue in the array
array Optional. The array forEach() was called upon "

So cur in your code is the current element of your array of integers. So it's an integer. So it has no value property, it IS the value you're looking for, as pointed out in a comment.

Answer 3

On the second iteration you wrongly put sum += cur.value . The right way shoud be: sum += cur .