JSFiddle - React, Tailwind, and code Playground

HTML

Not checked:&nbsp;<input type="checkbox"><br/>
Not checked:&nbsp;<input type="checkbox"><br/>
Checked:&nbsp;<input type="checkbox" class="myCheckBox"><br/>
Not checked:&nbsp;<input type="checkbox"><br/>
Not checked:&nbsp;<input type="checkbox"><br/>
Checked:&nbsp;<input type="checkbox" class="myCheckBox"><br/>
Checked:&nbsp;<input type="checkbox" class="myCheckBox"><br/>
Checked:&nbsp;<input type="checkbox" class="myCheckBox"><br/>

JavaScript

var elements = document.getElementsByClassName('myCheckBox');
for(var i = 0; i < elements.length; i++)
{
    elements[i].checked = true;
}