function sum(a) {
let currentSum = a;
function f(b) {
currentSum += b;
return f;
}
f.toString = function() {
return currentSum;
};
console.log(f);
return f;
}
alert( sum(1)(2) ); // 3
alert( sum(5)(-1)(2) ); // 6
please help me to understand the difference between - return f and f(). what happen with function code when activate return f? how it work? why console.log(f) return a number? i know f() return result, but return f? i dont understand.
In Javascript functions are first class objects. You can treat a function like any other variable or object, and pass them to functions, assign to other variables, and (as in this case) return them from functions.
A perhaps simpler example to show it might be something like
function foo() {
console.log("foo called");
}
bar = foo; // Assign the function foo to the variable bar
// Note that this doesn't actually call foo
bar(); // Now we call the foo function
My own example here is quite useless and only to show the principle. For a more useful example it's common for functions to return references to other functions, like in the example inside the question.
It happens to be that when you try to console.log any value it invokes "toString" method.
In your instance you override toString method instead of default implementation it returns a number
The function without the () is a pointer to the function. I use it with setTimeout all the time.
function doSomething() {
console.log('something');
}
setTimeout(doSomething, 5000);
Each time you invoke sum function, you are always returning reference of function f. So sum(1) will return the reference of f, while sum(1).toString() will return the 1 sum(1)(2) will return reference of f, while sum(1)(2).toString() will return 3
It is not recursion because you returning just the reference. So until you invoke it, the function is not called
