Using Index from Foreach in Other Array

Get loop counter/index using for…of syntax in JavaScript

for…in iterates over property names, not values, and does so in an unspecified order (yes, even after ES6). You shouldn’t use it to iterate over arrays. For them, there’s ES5’s forEach method that passes both the value and the index to the function you give it:

var myArray = [123, 15, 187, 32];

myArray.forEach(function (value, i) {
    console.log('%d: %s', i, value);
});

// Outputs:
// 0: 123
// 1: 15
// 2: 187
// 3: 32

Or ES6’s Array.prototype.entries, which now has support across current browser versions:

for (const [i, value] of myArray.entries()) {
    console.log('%d: %s', i, value);
}

For iterables in general (where you would use a for…of loop rather than a for…in), there’s nothing built-in, however:

function* enumerate(iterable) {
    let i = 0;

    for (const x of iterable) {
        yield [i, x];
        i++;
    }
}

for (const [i, obj] of enumerate(myArray)) {
    console.log(i, obj);
}

demo

If you actually did mean for…in – enumerating properties – you would need an additional counter. Object.keys(obj).forEach could work, but it only includes own properties; for…in includes enumerable properties anywhere on the prototype chain.

Looping through arrays and adding by index to another array

If I don't misunderstood your required output then you can use two foreach() to iterate every nested array and push every element by their index position of all nested array.

   $_GET = [
         "rentalPropertyAddress"=>["111 tree st","112 tree st","122 tree st"],
         "gasInitialized"=>["on","on","off"],
         "waterInitialized"=>["off","on","on"],
         "electricInitialized"=>["on","on","off"],
         "inspectionDate"=>["","",""],
         "rentalDate"=>["","",""],
         "vacantInitialized"=>["no","no","yes"]
];

$retval = [];
foreach ($_GET as $key => $item) {
    $i=0;
    foreach($item as $k=>$v){
      $retval[$i][] = $v;
      $i++;
    }  
}
echo "<pre>";
print_r($retval);
echo "</pre>";

DEMO: https://3v4l.org/LrpGo

How do you get the index of the current iteration of a foreach loop?

The foreach is for iterating over collections that implement IEnumerable. It does this by calling GetEnumerator on the collection, which will return an Enumerator.

This Enumerator has a method and a property:

MoveNext()
Current

Current returns the object that Enumerator is currently on, MoveNext updates Current to the next object.

The concept of an index is foreign to the concept of enumeration, and cannot be done.

Because of that, most collections are able to be traversed using an indexer and the for loop construct.

I greatly prefer using a for loop in this situation compared to tracking the index with a local variable.

Javascript getting an array from the index of another array

You could collect all indices in an object with the color as property.

This approach features

a classic for statement for iterating the index,
a logical nullish assignment ??= to check the property and assign an array if not given,
and Array#push, to get the index into the array with the color as name.

const
    colors = ['blue', 'red', 'red', 'red', 'blue', 'red', 'blue', 'blue'],
    indices = {};

for (let i = 0; i < colors.length; i++) {
    (indices[colors[i]] ??= []).push(i);
}

console.log(indices);

How to turn around the content of an array to another array with a forEach-Loop?

Ok so Issue is your forEach which not running with oldArray length mean 5 times
Why this is happening? It is because you are poping out value withing forEach which change the length and data of forEach.

You can check by putting console.log(oldArray); in forEach.

var arrayA = ["h", "e", "l", "l", "o"];var arrayB = [];
function copyArray(oldArray, newArray) {    var length = oldArray.length;    for (var i = 0; i < length; i++) {        var storeElement = oldArray.pop();        newArray.push(storeElement);    };    console.log(oldArray + " old array");    console.log(newArray + " new array");}

copyArray(arrayA, arrayB);

var reverseArray = ["h", "e", "l", "l", "o"].reverse();
console.log("reverse array");
console.log(reverseArray);

How can i get index use forEach loop?

You can simply use a for loop and use classList.contains() to check if the class is present. Also, you need to declare a local var, to store the index returned from the loop.

const product = document.querySelector(".product");
const productList = product.querySelectorAll("li");

function getIdx() {
  let idx;
  for (var i = 0; i < productList.length; i++) {
    let list = productList[i];

    if (list.classList.contains("on")) {
      idx = i;
    }
  }
  return idx;
};

function printIdx() {
  let idx = getIdx();

  console.log(idx);
}

productList.forEach(list => {
  list.addEventListener("click", printIdx);
});

<ul class="product">
  <li>aaaaa</li>
  <li>bbbbb</li>
  <li class="on">ccccc</li>
  <li>ddddd</li>
</ul>

How to find the foreach index?

foreach($array as $key=>$value) {
    // do stuff
}

$key is the index of each $array element