Javascript Scope in variables and functions

Question

I was given this snippet to debug in one of my interviews.

var module = (function sumModule(){
  
  var a = 0;
  var b = 0;
  
  const init = (a,b) =>{
    a = a;
    b = b;
  }
  
  function sum() {
    return a+b;
  }
  
  return {
  sum: sum,
  init
  }
  
})();

module.init(1,2);
console.log(module.sum())

The value being returned is 0 (0+0), the assignment of a and b in func init didn't overwrite the global var a and var b. Why is this so, can someone please explain?

NOTE: I was told to fix it without renaming the function parameters (a,b).

StackByMe · Accepted Answer · 2021-11-04 08:33:32Z

4

Instead of renaming the parameters you can also refer to the global variables via their namespace like you do with the init function.

*Or just rename the global variables, if that is allowed

var module = (function sumModule(){
  var a = 0;
  var b = 0;
  
  const init = (a, b) => {
    module.a = a;
    module.b = b;
  }
  
  function sum() {
    return module.a + module.b;
  }
  
  return {
    sum: sum,
    init,
  }
  
})();

module.init(1, 2);
console.log(module.sum())

answered Nov 4, 2021 at 8:33

StackByMe

6,57522 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Marc Over a year ago

This monkey patches just a & b to the module scope, and does not override the local/private a&b. Put console.log(module) before module.init() and after you call .sum and you see what i mean. a&b are not exisiting on the module object, you monkey patch them after the init call to the object. Private a&b are still 0.

Kien Nguyen · Accepted Answer · 2021-11-04 08:30:04Z

1

Because a and b in the assignments of init() function refers to its parameters, not the outer a and b. So those assignments don't affect values of the outer a and b.

edited Nov 4, 2021 at 8:30

answered Nov 4, 2021 at 8:24

Kien Nguyen

2,7212 gold badges9 silver badges25 bronze badges

Comments

Marc · Accepted Answer · 2021-11-04 16:41:47Z

Modified answer from @reyno. His example just monkey patch a & b to the module object after the .init call. Therefore this variables are public and not private anymore.

With this you get the same result, but maintain the variables a & b private.

To achieve this, use a private namespace object, like i did with private.

var module = (function sumModule(){

  var private = {
    a: 0,
    b: 0
  }
  
  const init = (a, b) => {
    private.a = a;
    private.b = b;
  }
  
  function sum() {
    return private.a + private.b;
  }
  
  return {
    sum: sum,
    init,
  }
  
})();

console.log(module) // no a or b property
module.init(1, 2);
console.log(module.sum())
console.log(module)  // still no a or b property

Collectives™ on Stack Overflow

Javascript Scope in variables and functions

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related