This calculator will only have the division, multiplication, addition, and subtraction operators but you can easily tack on more functions if you needed. You can also use keyboard number keys to calculate the value. Input type are included in the code for a form, which makes, ‘textfield’ and ‘button’. Every input field is specified with attribute value. The value attribute specifies the value of an <input> element. Eval() function, takes the expression and evaluates accordingly. ‘On click’ is the event. It occurs when the user clicks on an element.