JSFiddle - React, Tailwind, and code Playground

HTML

<h2>MOCK DATA</h2>
Stock name: <input id="stockNameMock" type="text" value="stock1" /><br/>
Stock price: <input id="stockPriceMock" type="number" value="10" /><br/>
Stock quantity: <input id="stockQuantityMock" type="number" value="5" /><br/>

<h2>INPUT</h2>
Site Name:
<select name="siteName" id="siteName">
  <option value="site1">site1</option>
  <option value="site2">site2</option>
  <option value="site3">site3</option>
</select><br/>
Stock name:
<select name="stockName" id="stockName">
  <option value="stock1">stock1</option>
  <option value="stock2">stock2</option>
  <option value="stock3">stock3</option>
</select><br/>
Quantity:
<input type="number" name="quantity" id="quantity" value="0" /><br/>
<button type="button" id="submit">Submit</button>
<br/>
<br/>
<div>Status: <span id="statusMessage">-</span></div>

CSS

#statusMessage {
  color: red;
}

JavaScript

var mockData = {
		stockName: null,
  	stockPrice: 0,
  	stockQuantity: 0
}

function setMockData() {
    mockData = {
      	stockName: $('#stockNameMock').val(),
      	stockPrice: parseInt($('#stockPriceMock').val(), 10) || 0,
      	stockQuantity: parseInt($('#stockQuantityMock').val(), 10) || 0
  	}
}

function compareData() {
    var siteName = $('#siteName').val(), // Get the selected site name
    		stockName = $('#stockName').val(), // Get the selected stock name
    		quantity = parseInt($('#quantity').val(), 10) || 0; // Get the selected quantity
    
    setMockData(); // Set mock data

    $.ajax({
        url: '/echo/json/', // Replaced for mocking
        type: 'post',
        async: true,
        data: {
            json: JSON.stringify(mockData) // Added for mocking
        },
        dataType: 'json',
        success: function(data) {
        		if(data.stockName === stockName) {
            		if(data.stockQuantity < quantity) {
                    $('#statusMessage').text('Stock quatity to low');
                } else {
                    $('#statusMessage').text('-');
                }
            } else {
            		$('#statusMessage').text('Stock name mismatch');
            }
        }

    });
}

$('#submit').on('click', compareData);

setMockData();