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())

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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).

CodePudding user response：

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())

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CodePudding user response：

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.

CodePudding user response：

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

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