JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parentdiv">
    <textarea id="ta1" class="childtextarea"></textarea>
</div>

CSS

.parentdiv > .childtextarea {
    width: 570px;
}

.parentdiv > .someotherclass {
    width: 200px;
}

JavaScript

var getCSS = function (prop, fromClass) {
 
    var $inspector = $("<div>").css('display', 'none').addClass(fromClass);
    $("body").append($inspector); // add to DOM, in order to read the CSS property
    try {
        return $inspector.css(prop);
    } finally {
        $inspector.remove(); // and remove from DOM
    }
};

var getCSS2 = function (prop, fromClass, $sibling) {
 
    var $inspector = $("<div>").css('display', 'none').addClass(fromClass);
    if ($sibling != null) {
        $sibling.after($inspector);
    } else {
        $("body").append($inspector); // add to DOM, in order to read the CSS property
    }
    try {
        return $inspector.css(prop);
    } finally {
        $inspector.remove(); // and remove from DOM
    }
};


// now call it
alert(getCSS('width', 'childtextarea'));
alert(getCSS2('width', 'childtextarea', $('#ta1')));
alert(getCSS2('width', 'someotherclass', $('#ta1')));