JSFiddle - React, Tailwind, and code Playground

JavaScript

var sum = (function() {		
    var currentSum;
    
    return function(a) {
        if(a === undefined) {
          return currentSum
        }
        
				currentSum = a;
        
        function f(b) {
            currentSum += b;
            return f;
        }

        f.toString = function() {
            return currentSum;
        };

        return f;
    }
}())

alert( sum(1)(2) ); // 3
alert( sum(5)(-1)(2) ); // 6
alert(sum()); // 6