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;
}