Add up values for array key inside a loop in javascript

Question

Total noob question. In PHP I can easily do this:

foreach( $array1 as $key => $value ) {
    $array2[$key] += $value;
}

I cannot figure out a way to do this in javascript... I'm sure there must be a way.

EDIT: It doesn't really matter what I loop over, there should be a generic solution for creating an associative array or object on the fly inside a loop and also dynamically create it's key/value pairs with the option to add up numbers. Maybe the following piece of code will help to understand:

var vat = {};
InvoiceItems.each(function(item){

    vat_rate = item.get('vat_rate');
    vatsum = Number(roundNumber( 100 * item.get('vat'), 2 ) / 100, 2);
    vat[vat_rate] += vatsum;

});

The problem is, this results in an object like this: {"20": "undefined18.00","null":"0.00"}

So there's the "undefined" and also an empty null key/value pair.

Answer 1

JavaScript does not have such a for-each loop, as defined in PHP. If you're dealing with arrays, you should use a for( ; ; ) loop. Otherwise, if you're dealing with objects, the closest you can get is:

// Assume `object` to exist
var key, value; // Declare variables
for (key in object) {
    value = object[key];
}

Removed part of the original answer, because it seems not relevant to the question

Code, as a response to the comment:

var dom_elements = document.querySelectorAll("input"); // Example, HTMLCollection
var array2 = {};   // Holds counters
for (var i=0; i<dom_elements.length; i++) {
    var element = dom_elements[i]; // Select element
    var key = element.name;        // Example: Elements are grouped by name
    var value = element.value;     // Example: value
    if (key in array2) {
        array2[key] += value;
    } else {                  // If the key does not exist, create one:
        array2[key] = value;
    }
}

Update 2: Based on updated question

Replace vat[vat_rate] += vatsum; with:

if (vat_rate in vat) {
    vat[vat_rate] += vatsum;
} else {
    vat[vat_rate] = vatsum;
}

Answer 2

I think what you are trying to do is this:

var array2 = [4, 5, 6];
var array1 = [1, 2, 3];
for (var key in array1) {
    array2[key] += array1[key];
}

This will add each item in array1 to its corresponding item in array2 and replace the item in array2 with the result, which is essentially what you are doing in your PHP code.