Function closures

Question

Hi I'm trying to run the following function:

 function add (a, b) {
 return a + b;
 }
 var make_lazy = function (add, a, b) {
 return function () {
 add(a,b);
 }
 }

Basically what I'm trying to do is to pass another function as an argument and its parameters to the make_lazy function - and then run the function that was passed in as the argument along with the other two parameters. I get undefined is not function as an error when I try to run the code.

Answer 1

You forgot the return statement in the anonymous function that you're returning from make_lazy:

var make_lazy = function (add, a, b) {
 return function () {
 return add(a,b) // <----- here
 }
}

Answer 2

I think you are trying for something like this.

function add (a, b) {
 return a + b;
}
 var make_lazy = function (a, b) {
 return function () {
 return add(a,b);
 }
}

Then you can call var lazy = make_lazy(3,5); and later call lazy() to get 8

• Touffy: I think you missed the main issue (see my answer). But it would indeed be silly to pass add if it's the only function you're ever going to pass.
• Subin Sebastian: I dont understand. Dont you see return keyword in question's make_lazy function?

Answer 3

Wrap you lazy function body inside a self calling function and return.

function add (a, b) {
 return a + b;
 }
 var make_lazy = function (add, a, b) {
 return (function () {
 add(a,b);
 })();
 }

• Touffy: You're missing the point of making a lazy function. That means he wants to run it only if he has to. Presumably the "add" function is an example and in his real code it's much more expensive.

Answer 4

Here's my point of view.

When you assign a function to make_lazy variable, after that you should make an invocation make_lazy() with the same params as they were in the definition of that function:

make_lazy(function expression, a, b);

This portion:

function (add, a, b)

just makes add a local variable, this is not the same as add(a,b) which is defined above.

To make the code work, try to invoke make_lazy as

make_lazy(add, 3, 4)