Why calling a function inside another function doesn't work?

Question

var mult = (function(){
 var cache = {};
 var calculate = function(){
 var a = 1;
 for(var i = 0, l = arguments.length; i < l; i++){
 a = a * arguments[i];
 }
 return a;
 }
 return function(){
 return calculate(arguments);
 }
})();
console.log(mult(1, 2));

Above is my code, I expect the mult function will give me value 2, but instead it outputs NaN. I changed the line calculate(arguments) to caculate.apply(null, arguments) and it worked. I don't know why the old code doesn't work? Why do I need to use apply in this case? What does null represent here?

• Paul: Why are you going for this much indirection anyway?
• ventaquil: Why this code is so much complicated?
• charlietfl: try return calculate(...arguments);

Answer 1

Your calculate function wants separate arguments, but you passed in an array ¹. Using .apply spreads the content of the array for you.

¹ Technically an array-like arguments object that does not inherit from Array.

• Paul: Well, technically he passed in the array-like arguments object, but close enough. ;)
• Derek 朕會功夫: You are absolutely correct, but using the word "array" is easier to understand than "array-like arguments object" :)
• user47589: Marginally easier to understand, but less accurate.
• Paul: I know, I just wanted to be pedantic to pre-empt follow up questions about why certain array functions weren't working.
• CacaJ: but why did it use apply.(null..)? what does null represent?