Develop Reference

Jquery - Variable not updating into another variable

first of all i'm not a programmer of any kind, so i please need you to fix my issue.
I have a contact form which have inside a place where i can add more input fields till a maximum of 10.
Each field i add it has inside the code a class that i called "pers", and i want that this class pers has an incremental number near to it, as pers1, 2, 3 and so go on.
Getting the incremental value was easy but the problem is that the class "pers" wont update the variable keeping the first number on document load.
Here is the code
<script type="text/javascript">
window.s = 1;
</script>
<script type="text/javascript">
$(document).ready(function() {
var addButton = $('.add_button');
var wrapper = $('.field_wrapper');
var maxField = 10;
var fieldHTML = '<div style="margin-top:5px;"><input type="text3" name="pers' + window.s + '" placeholder="Nome e Cognome"/><input type="time" style=margin-left:3.5px; autocomplete="off"><input type="time" style=margin-left:3.5px; autocomplete="off"><img src="IMG/less_icon.png"></div>';
$(addButton).click(function() {
if (window.s < maxField) {
window.s++; //Increment field counter
$(wrapper).append(fieldHTML); //Add field html
}
});
$(wrapper).on('click', '.remove_button', function(e) {
e.preventDefault();
$(this).parent('div').remove(); //Remove field html
s--; //Decrement field counter
});
});
</script>
There is a global variable "window.s" because it was my last try to get the var updated.
The fields adds correctly till 10 as it should be, but the "window.s" or when it was only "s" still keeps his first value of 1, and so "pers" is always "pers1". I thought that exposing a global variable i would have fix the problem, but nothing.
What am i doing wrong?
Thank you very much for your help guys.

The problem with your code is because you only set fieldHTML once, when the page loads. At this point window.s is 1, so this is the value that's used every time you reference fieldHTML. The quick fix to this would be to put the fieldHTML definition inside the click() event handler.
However the better approach it to entirely remove the need for the incremental variable at all. Use the same name on all the fields you dynamically generate. This way you don't need to maintain the count (eg. if there have been 5 added and someone deletes the 3rd one, you'll currently end up with 2 per5 elements). Because of this it also simplifies the JS logic. In addition you should put the HTML to clone in a <template> element, not the JS, so that there's no cross-contamination of the codebase.
Here's a working example of the changes I mention:
jQuery($ => {
var $addButton = $('.add_button');
var $wrapper = $('.field_wrapper');
var maxFields = 10;
var fieldHTML = $('#field_template').html();
$addButton.click(() => {
if ($('.field_wrapper > div').length < maxFields) {
$wrapper.append(fieldHTML);
}
});
$wrapper.on('click', '.remove_button', e => {
e.preventDefault();
$(e.target).closest('div').remove();
});
});
.field_wrapper>div {
margin-top: 5px;
}
.field_wrapper input.time {
margin-left: 3.5px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<button class="add_button">Add</button>
<div class="field_wrapper"></div>
<template id="field_template">
<div>
<input type="text3" name="pers[]" placeholder="Nome e Cognome" />
<input type="time" name="time1[]" class="time" autocomplete="off" />
<input type="time" name="time2[]" class="time" autocomplete="off" />
<a href="#" class="remove_button" title="Rimuovi">
<img src="IMG/less_icon.png">
</a>
</div>
</template>
You can then receive all the input values as an array in your PHP code, like this.

It's because you have not used the updated value there.
First, you assign a value to window.s, and then you create a variable that uses the value of window.s from the initial state, and on each addition, it just appends the same fieldHtml. So, you are getting the same value.
Here is the answer, you are looking:
<div><button class="add_button">click me</button></div>
<div class="field_wrapper"></div>
<script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
<script type="text/javascript">
window.s = 1;
</script>
<script type="text/javascript">
$(document).ready(function() {
var addButton = $('.add_button');
var wrapper = $('.field_wrapper');
var maxField = 10;
$(addButton).click(function() {
if (window.s < maxField) {
window.s++; //Increment field counter
let fieldHTML = generateLine(window.s);
$(wrapper).append(fieldHTML); //Add field html
}
});
$(wrapper).on('click', '.remove_button', function(e) {
e.preventDefault();
$(this).parent('div').remove(); //Remove field html
s--; //Decrement field counter
});
});
function generateLine(lineNumber){
return '<div style="margin-top:5px;"><input type="text3" name="pers' + lineNumber + '" placeholder="Nome e Cognome"/><input type="time" style=margin-left:3.5px; autocomplete="off"><input type="time" style=margin-left:3.5px; autocomplete="off"><img src="IMG/less_icon.png"></div>';
}
</script>

Increment and update value in the total number after insert new rows dynamically

EDIT: I have updated the code with the answers.
I have a increment function that is working fine. However:
1. I would like to set some limits based on the total number available in one of the span. For example, 10. So the incrementing can't be more than 10. #DONE
Another issue is that I am planning to have multiple rows and before I save I want to make sure if we count the increments in every row it should not be more than 10 as well. If it decrease the total number (span) dynamically would be nice.
I'm adding rows dynamically with the ADD button, how can I add news rows that actually work with the current functions? Mine rows just clone the first one and the increment function is disabled.
document.addEventListener('DOMContentLoaded', async function() {
document.querySelector('#addlocationdest').addEventListener('click', add);
});
function add() {
var x = 1;
var container = document.getElementById('destination');
var detail = document.getElementById('row');
var clone = detail.cloneNode(true);
clone.id = "destination" + x;
x++;
container.appendChild(clone);
}
window.addEventListener("load", () => {
let elTotalQuantity = document.querySelector("#totalqty");
let totalQuantity = parseInt(elTotalQuantity.innerHTML);
function getSumOfRows() {
let sum = 0;
for (let input of document.querySelectorAll("form .row > input.quantity"))
sum += parseInt(input.value);
return sum;
}
for (let row of document.querySelectorAll("form .row")) {
let input = row.querySelector("input");
row.querySelector(".increment").addEventListener("click", () => {
if (getSumOfRows() >= totalQuantity) return;
input.value++;
elTotalQuantity.innerHTML = totalQuantity - getSumOfRows();
});
row.querySelector(".decrement").addEventListener("click", () => {
if (input.value <= 0) return;
input.value--;
elTotalQuantity.innerHTML = totalQuantity - getSumOfRows();
});
}
});
<div id="location" class="hide">
<div class="title">Transfer details</div><br>
<div class="line padded-s">Total Quantity: <span>10</span></div>
<br>
<form>
<label>New Total Quantity at this location: <span id="totalqty">10</span></label>
<br>
<div id="destination">
<div id="row" class="row">
<button type="button" class="decrement">-</button>
<input type="text" class="quantity" value="0" readonly/>
<button type="button" class="increment">+</button>
<a>Location: </a>
<input type="text" class="location" value="0" readonly/>
</div>
</div>
</form>
<label>Total being transfer: <p id="total-sum"></p></label>
<br>
<button type="button" id="addlocationdest">ADD</button>
<button type="button" id="removelocationdest">REMOVE</button>
</div>

Prologue
As long as the total quantity is fixed at the beginning of the script-execution, this works. Otherwise, it would be best to save the actual allowed total quantity as an attribute, and observe it using a MutationObserver. That way you can update your max. value in your code dynamically, when the total quantity-attribute changes. You can define custom attributes by naming them "data-*" where "*" is a custom name.
Solution for your problem
You are using the same ID on multiple elements. What you meant were classes, so change id="increment" to class="increment", and the same for decrement.
Since we don't want to input something with the buttons, but add listener to them, I'd say it is better to actually use <button>. In forms, buttons act as type="submit", which we don't want, so we need to change it to type="button".
Since the rows and the total quantity actually belong together, it is wiser to place them together into one <form>-element. However, you can still group the buttons and inputs as a row together using <div>.
Now regarding the in-/decrementing of the row's values and the total quantity:
Save the allowed total quantity in a variable
Add event-listener to the corresponding buttons
If action is valid, change row's value
Update total quantity number to totalQuantity - getSumOfRows()
To add new rows dynamically, we create and setup such an element, and append it to the form. See the appendNewRow()-function below.
Sidenote
I have added the readonly attribute to the input-fields so that you cannot enter numbers via keyboard.
window.addEventListener("load", () => {
let elTotalQuantity = document.querySelector("#totalqty");
let totalQuantity = parseInt(elTotalQuantity.innerHTML);
function getSumOfRows() {
let sum = 0;
for (let input of document.querySelectorAll("form .row > input.quantity"))
sum += parseInt(input.value);
return sum;
}
function updateTotalQuantity() {
elTotalQuantity.innerHTML = totalQuantity - getSumOfRows();
}
function appendNewRow() {
let row = document.createElement("div");
row.classList.add("row");
let child;
// input.quantity
let input = document.createElement("input");
input.classList.add("quantity");
input.value = "0";
input.setAttribute("readonly", "");
input.setAttribute("type", "text");
row.append(input);
// button.increment
child = document.createElement("button");
child.classList.add("increment");
child.innerHTML = "+";
child.setAttribute("type", "button");
child.addEventListener("click", () => {
if (getSumOfRows() >= totalQuantity) return;
input.value++;
updateTotalQuantity();
});
row.append(child);
// button.increment
child = document.createElement("button");
child.classList.add("decrement");
child.innerHTML = "-";
child.setAttribute("type", "button");
child.addEventListener("click", () => {
if (input.value <= 0) return;
input.value--;
updateTotalQuantity();
});
row.append(child);
// button.remove-row
child = document.createElement("button");
child.classList.add("remove-row");
child.innerHTML = "Remove";
child.setAttribute("type", "button");
child.addEventListener("click", () => {
row.remove();
updateTotalQuantity();
});
row.append(child);
document.querySelector("form .rows").append(row);
}
document.querySelector("form .add-row").addEventListener("click", () => appendNewRow());
appendNewRow();
});
<form>
<label>Total Quantity: <span id="totalqty">10</span></label>
<br>
<div class="rows">
</div>
<button type="button" class="add-row">Add new row</button>
</form>

QuerySelector only selects the first occurrence so you haven't really added a listener to the second "row". You should use querySelectorAll but, instead of unique ids, use classes.
<input class="increment" type="button" value="+" />
Now you can use document.querySelectorAll(".increment") to get all elements in an array.
You can traverse in the DOM by using parentElement. By knowing which button you clicked, you can traverse up to the form element and then select the first child - which is an input. A more dynamic way would be to use querySelector to select the input, in case the HTML change in the future. Anyway, that's how you can know which input to manipulate based on where the buttons are in the DOM.
I added two global variables, totalSum and maxSum. maxSum is fetched from your span element (which I assigned an unique id to). totalSum makes sure that all inputs combined doesn't exceed maxSum.
You had some duplicate code, so I refactored it into a new method: changeValue.
In all, I think the code speaks for itself.
Oh, this code doesn't take into account that the user can change the value inside the input. I will leave that for you to figure out with an "oninput" listener on each text input.
var totalSum = 0; // 3
var maxSum = 0
var totalSumElement = null;
document.addEventListener('DOMContentLoaded', async function() {
totalSumElement = document.getElementById('total-sum');
maxSum = document.getElementById('max-sum').innerText;
var incrementElements = document.querySelectorAll('.increment'); // 1
var decrementElements = document.querySelectorAll('.decrement');
addListener('click', incrementElements, incrementValue);
addListener('click', decrementElements, decrementValue);
});
function addListener(type, elementArr, func) {
for (element of elementArr) {
element.addEventListener(type, func);
}
}
function withinRange(newValue) {
var maxReached = newValue > maxSum; // 3
var zeroReached = newValue < 0;
return !maxReached && !zeroReached;
}
function changeValue(event, change) { // 4
if (withinRange(totalSum + change)) {
let parent = event.currentTarget.parentElement; // 2
let input = parent.children[0];
let value = parseInt(input.value) || 0;
if (withinRange(value + change)) {
input.value = value + change;
totalSum = totalSum + change;
}
}
totalSumElement.textContent = `Total: ${totalSum}`;
}
function incrementValue(event) {
changeValue(event, 1);
}
function decrementValue(event) {
changeValue(event, -1);
}
#totalqty {
padding-bottom: 1rem;
}
<div id="totalqty" class="line padded-s">Total Quantity: <span id="max-sum">10</span></div>
<form>
<input type="text" value="0" />
<input class="increment" type="button" value="+" />
<input class="decrement" type="button" value="-" />
</form>
<form>
<input type="text" value="0" />
<input class="increment" type="button" value="+" />
<input class="decrement" type="button" value="-" />
</form>
<p id="total-sum"></p>

Unique name for Add More input

I am trying to create an add more button which will create a new input field. However, I would like to have an unique name set for it.
I tried to search up for an answer, but this does not answer my question.
So, basically what I tried to make my namefield unique is to use the php method rand(). The concept is that - when the add more button is clicked, it will have a name attached to the number given to me by rand().
However, what happens is that it takes the value generated by rand() and applies it to all the names of all the inputs generated.
This is my code and what I tried:
HTML:
<div class="field_wrapper">
<div>
<input type="text" name="field_name[<?php echo rand(); ?>]" value=""/>
Add More
</div>
</div>
JQUERY / JAVASCRIPT:
<script type="text/javascript">
$(document).ready(function(){
var maxField = 100; //Input fields increment limitation
var addButton = $('.add_button'); //Add button selector
var wrapper = $('.field_wrapper'); //Input field wrapper
var fieldHTML = '<div><input type="text" name="field_name[<?php echo rand(); ?>]" value=""/>Remove</div>'; //New input field html
var x = 1; //Initial field counter is 1
//Once add button is clicked
$(addButton).click(function(){
//Check maximum number of input fields
if(x < maxField){
x++; //Increment field counter
$(wrapper).append(fieldHTML); //Add field html
}
});
//Once remove button is clicked
$(wrapper).on('click', '.remove_button', function(e){
e.preventDefault();
$(this).parent('div').remove(); //Remove field html
x--; //Decrement field counter
});
});
</script>
As you can see, the first field generates the number as intended. If you click on the add more, the second field does create an unique number. However, if you click add more once again, the third field copies the same name as the 2nd field.
How do I go about achieving what I want and why is rand() not generating a new code?
Also, does rand() guarantee me that it will be an unique ID or is there a chance for it to repeat the same number?
If it does repeat, then what would be the best approach to take to make it as unique as possible?

If you generate random name with PHP it is done once on the server. Your JS code then copies the same element. What you need is to generate unique names with js.
Avoid random if you can, theoretically, you can hit the same number and run into mysterious bugs.
var generateField = function(name)
{
return '<div><input type="text" name="'+name+'" value=""/>Remove</div>'; //New input field html
}
//Once add button is clicked
$(addButton).click(function(){
//Check maximum number of input fields
if(x < maxField){
x++; //Increment field counter
$(wrapper).append(generateField('field_name['+x+']' ) ); //Add field html
}
});

Random does not necessarily mean unique, even if collisions would be extremely rare. This solution simply increments a totalFieldsCreated variable to get the next unique number (up to the maximum value JavaScript can provide.)
The new fields are created dynamically instead of using a fixed string of HTML. (This technique is more flexible.)
$(document).ready(function() {
// Defines global identifiers
let
currentFieldCount = 1,
totalFieldsCreated = 1;
const
maxFieldCount = 100,
addButton = $('.add_button'),
wrapper = $('.field_wrapper');
// Calls `addField` when addButton is clicked
$(addButton).click(addField);
// Executes anonymous function when `Remove` is clicked, which removes
// the parent div, and decrements (and logs) `currentFieldCount`
$(wrapper).on('click', '.remove_button', function(e) {
e.preventDefault();
$(this).parent('div').remove();
currentFieldCount--;
console.log(`currentFieldCount: ${currentFieldCount}`);
});
// Defines the `addField` function
function addField(){
// Makes sure that `currentFieldCount` and `totalFieldsCreated`
// are not at maximum before proceeding
if(
currentFieldCount < maxFieldCount &&
totalFieldsCreated < Number.MAX_VALUE
){
// Creates an input element, increments `totalFieldsCreated`,
// and uses the incremented value in the input's `name` attribute
const input = document.createElement("input");
input.type = "text";
input.name = "field" + ++totalFieldsCreated;
input.value = "";
// Creates an anchor element with the `remove_button` class
const a = document.createElement("a");
a.href = "javascript:void(0);";
a.classList.add("remove_button");
a.title = "remove";
a.innerHTML = "Remove";
// Adds the new elements to the DOM, and increments `currentFieldCount`
const div = document.createElement("div");
div.appendChild(input);
div.appendChild(a);
$(wrapper).append(div);
currentFieldCount++;
// Logs the new values of both variables
console.log(
`currentFieldCount: ${currentFieldCount},`,
`totalFieldsCreated ${totalFieldsCreated}`
);
}
}
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="field_wrapper">
<div>
<input type="text" name="field1" value="" />
Add More
</div>
</div>

Try Math.random() in js rather than rand() in php ,Math.floor(Math.random()*90000) + 10000 will generate a five digit random number , Hope this helps
$('.rand').attr('name',"fields["+Math.floor(Math.random()*90000) + 10000+"]")
$('.add_button').click(function(e){
$('.field_wrapper').append('<div><input type="text" name=fields['+Math.floor(Math.random()*90000) + 10000+'] value=""/>Remove</div>')
})
$(document).on('click','.remove_button',function(e){
$(this).parent().remove()
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.0/jquery.min.js"></script>
<div class="field_wrapper">
<div>
<input type="text" class="rand" value=""/>
Add More
</div>
</div>

Adding dynamic form fields with JavaScript

I have a form that is used to create a JSON array, see my previous question for reference.
In this form a user can add additional details to the fom by clicking a button and filling in said extra details.
These would then be placed into an array in a similar fashion to the below:
<input type="text" name="AdditionalCitizenship[0][CountryOfResidency]">
<input type="text" name="AdditionalCitizenship[0][TaxIdentificationNumber]">
<input type="text" name="AdditionalCitizenship[1][CountryOfResidency]">
<input type="text" name="AdditionalCitizenship[1][TaxIdentificationNumber]">
This would allow me to grab as many details as the user entered by incrementing the array index.
I was handed this script to add extra form fields.
$(document).ready(function() {
var max_fields = 10; //maximum input boxes allowed
var wrapper = $(".input_fields_wrap_tel"); //Fields wrapper
var add_button = $(".add_field_button_tel"); //Add button ID
var x = 1; //initlal text box count
$(add_button).click(function(e){ //on add input button click
e.preventDefault();
if(x < max_fields){ //max input box allowed
x++; //text box increment
$(wrapper).append('<div><div class="row"><div class="form-group col-md-4"><label for="AdditionalTelephoneType">Telephone Type</label><input type="text" class="form-control" name="AdditionalTelephoneType[]" ></div><div class="form-group col-md-4"><label for="AdditionalTelephoneDialingCode">Dialing Code</label><input type="text" class="form-control" name="AdditionalTelephoneDialingCode[]"></div><div class="form-group col-md-4"><label for="AdditionalTelephoneNumber">Telephone Number</label><input type="text" class="form-control" name="AdditionalTelephoneNumber[]" ></div></div>Remove</div>'); //add input box
}
});
$(wrapper).on("click",".remove_field", function(e){ //user click on remove text
e.preventDefault(); $(this).parent('div').remove(); x--;
})
});
I am trying to use as is but in this scenario, it is difficult to increment x within the created HTML as it seems to blow up the function.
Could I create the HTML more iteratively like so:
First, create the DIV structure as a wrapper like:
var html = "<div></div>"
Then append an input to this variable called input
var input = document.createElement("input");
input.type = "text";
input.name = "AdditionalTelephoneType[" + x"]";
... and then insert the whole HTML block by using wrapper.append with the variables I have created previously?

You can find the highest x dynamically, see comments:
$("#add").on("click", function() {
// Get the containing form
var form = $(this).closest("form");
// Get all the AdditionalCitizenship fields from it using ^=, see
// https://www.w3.org/TR/css3-selectors/#attribute-substrings
var fields = form.find("input[name^=AdditionalCitizenship]");
// Find the one with the highest [x]
var x = fields.get().reduce((x, element) => {
var thisx = element.name.match(/AdditionalCitizenship\[(\d+)\]/);
if (thisx) {
thisx = +thisx[1]; // The capture group, convert to number
if (x < thisx) {
x = thisx;
}
}
return x;
}, 0);
// Add one
++x;
// Use x
console.log("Next x is " + x);
form.append('<input type="text" name="AdditionalCitizenship[' + x + '][CountryOfResidency]">');
form.append('<input type="text" name="AdditionalCitizenship[' + x + '][TaxIdentificationNumber]">');
});
<form>
<input type="text" name="AdditionalCitizenship[0][CountryOfResidency]">
<input type="text" name="AdditionalCitizenship[0][TaxIdentificationNumber]">
<input type="text" name="AdditionalCitizenship[1][CountryOfResidency]">
<input type="text" name="AdditionalCitizenship[1][TaxIdentificationNumber]">
<input type="button" id="add" value="Add">
</form>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

JS - Attach the same function to many html forms

I am trying to make an e-commerce-like webpage (for practice) wherein a click on any of the buttons would update the cart value by the number (quantity) specified on the input element.
So far, I was only able to update the cart from the first form because when I try to assign the function on every form using a loop, the cart updates for a millisecond then returns to zero. I assume its because of the scope.
I know there's an easier way to do this without manually assigning the function for every document.forms[n]
JS
window.onload = function()
{
var getForm = document.forms[0];
var numItems = 0;
getForm.onsubmit = function(event)
{
event.preventDefault();
var getInput = getForm.elements["num-item"].value;
if(parseInt(getInput))
{
numItems = numItems + parseInt(getInput);
var getCart = document.getElementById("item-count");
getCart.innerHTML = numItems;
getForm.reset();
}
else
{
alert("Please enter a valid number");
}
}
HTML
Cart:
<div class="basket">
<p><i class="fa fa-shopping-basket"></i></p>
<p id="item-count">0</p>
</div>
HTML Form: For brevity, I'm only posting 1 form example, but in reality, I have 6 other forms that are exactly the same.
<div class="buy-config">
<form class="buy-form" name="buy-form">
<label>Quantity:</label>
<input type="text" class="num-item" />
<button class="buy-btn">Add to Cart</button>
</form>
</div>

Loop through all of the forms by querying the selector (using whatever method you prefer, depending on performance requirements and markup flexibility -- I've used getElementsByClassName) and executing a for loop.
Inside the loop, bind a function to the "submit" event using addEventListener. You can define the function in-line (as I've done), or define the function elsewhere, assign it to a variable, and reference the variable when binding to the event.
Within the event listener function, you will refer to the form that was submitted as this.
On top of the changes described above, I've made some minor changes to your code:
Your previous version was overwriting the contents of the cart each time. This may have been on purpose, depending on whether you have one "basket" for each item or one overall (this wasn't clear in the question). So, rather than initialize numItems to zero, I've initialized it to the current number of items in the cart.
Consider using input type="number" HTML form elements. They're supported by nearly every browser and only accept digits -- they also have up/down arrows and can be set with the scroll wheel. On browsers that don't support them, they fall back to a basic text input.
var forms = document.getElementsByClassName("buy-form");
for (var i = 0; i < forms.length; i++) {
forms[i].addEventListener("submit", function(event) {
event.preventDefault();
var numItems = parseInt(document.getElementById("item-count").innerHTML);
var getInput = this.getElementsByClassName("num-item")[0].value;
if (parseInt(getInput)) {
numItems = numItems + parseInt(getInput);
var getCart = document.getElementById("item-count");
getCart.innerHTML = numItems;
this.reset();
} else {
alert("Please enter a valid number");
}
});
}
<div class="basket">
<p><i class="fa fa-shopping-basket"></i></p>
<p id="item-count">0</p>
</div>
<div class="buy-config">
<form class="buy-form" name="buy-form">
<label>Quantity:</label>
<input type="number" class="num-item" />
<button class="buy-btn">Add to Cart</button>
</form>
</div>
<div class="buy-config">
<form class="buy-form" name="buy-form">
<label>Quantity:</label>
<input type="number" class="num-item" />
<button class="buy-btn">Add to Cart</button>
</form>
</div>
<div class="buy-config">
<form class="buy-form" name="buy-form">
<label>Quantity:</label>
<input type="number" class="num-item" />
<button class="buy-btn">Add to Cart</button>
</form>
</div>

You can use the jQuery selector.
<script src="http://code.jquery.com/jquery-1.12.0.min.js"></script>
<script>
$(document).ready(function() {
$('.buy-btn').click(function(){
$(this).parent('form').submit();
});
});
</script>
<form class="buy-form">
<label>Quantity:</label>
<input type="text" class="num-item" />
<button class="buy-btn">Add to Cart</button>
</form>
The code above will setup a function for each HTML elements that has the css class buy-btn.
You can select anything using parent, children, prev, next or find function from jQuery.
Of course this is just a basic exemple I'm showing here, and again some simple example could be :
$('.buy-btn').click(function(){
$(this).parent('form').submit();
//var itemCount = $('#item-count').html();
//itemCount++;
//$('#item-count').html(itemCount);
var numItem = $(this).prev('.num-item').val();
$('#item-count').html(numItem);
});

Unfortunately, you're going to have to loop through the elements in your JavaScript and assign the function to each, however you can do it a bit simpler with some querySelector methods thrown in:
window.onload = function() {
var getCart = document.getElementById('item-count');
var forms = document.querySelectorAll('.buy-form');
var numItems = 0;
var isNum = function(n) {
return(!isNaN(parseFloat(n)) && isFinite(n));
};
var handler = function(e) {
(e || event).preventDefault();
var getInput = this.querySelector('.num-item').value;
if(isNum(getInput)) {
numItems += parseInt(getInput);
getCart.innerHTML = numItems;
this.reset();
} else {
alert("Please enter a valid number");
}
};
for(var i = 0, len = forms.length; i < len; i++) {
forms[i].addEventListener('submit', handler);
}
};