JSFiddle - React, Tailwind, and code Playground

HTML

<!-- Displays -->
        <input type="text" name="display" id="display" value="0" class="display">
        <input type="text" name="subdisplay" id="subdisplay" class="subdisplay" readonly>

<!-- Calculating operations -->
        <input type="button" onclick="print_equal()" id="equal" value="=">
        <input type="button" onclick="num_add()" id="plus" value="+">

<!-- Reset -->
        <input type="button" value="C" onClick="reset()" class="reset">

JavaScript

var number = 0;													//the result
var operation = ' ';										    //the chosen calculating operation
var temp_Val = 0;												//the last entered value (for the subdisplay)

var print_equal = function () {
    var displayVal = document.getElementById("display");		
    displayVal.value = number;									
};

var num_add = function () {
    var displayVal = document.getElementById("display");		
	temp_Val = displayVal.value;								//saves the value of the display (for the subdisplay)
    console.log(temp_Val);										//schreibt den Wert des Displays auf die Konsole
	number += parseFloat(displayVal.value);						//calculates the result
	operation = '+';											//saves the used operation (for the subdisplay)
	print_subdisplay();											//runs the function that's building the value of the subdisplay
	displayVal.value = "";										//resets the main display
};

var print_subdisplay = function () {
	var subdisplayVal = document.getElementById("subdisplay");
	subdisplayVal.value = temp_Val + operation;    //creates a String with both the first entered value and the operation
};

function reset() {
	number = 0;
	operation = ' ';
    var displayVal = document.getElementById("display");
    displayVal.value = "";
}