JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" value="click me!" onclick="clicked()" />
<form id="MyForm">
<input type="text" name="array[first_1][second_1]" value="123" />
<input type="text" name="array[first_2][second_2]" value="456" />
<input type="text" name="array[first_3][second_3]" value="789" />
</form>

JavaScript

window.clicked = function  () {
    var MyForm = document.getElementById('MyForm');
    for (var n = 0; n < MyForm.length; ++n) {
        var current = MyForm.elements[n],m,
             name=current.getAttribute('name');    
        if(m=name.match(/([^[]+?)\[(.*?)\]\[(.*?)\]/)){
           document.body.innerHTML += '<p>'+m[1]+'['+m[2]+']['+m[3]+']='+current.value;
        }
    }
}