Node Modules - exporting a variable versus exporting functions that reference it?

Easiest to explain wi开发者_如何学Cth code:

##### module.js
var count, incCount, setCount, showCount;
count = 0; 

showCount = function() {
 return console.log(count);
};
incCount = function() {
  return count++;
};
setCount = function(c) {
  return count = c;
 };

exports.showCount = showCount;
exports.incCount = incCount;
exports.setCount = setCount; 
exports.count = count; // let's also export the count variable itself

#### test.js
var m;
m = require("./module.js");
m.setCount(10);
m.showCount(); // outputs 10
m.incCount();  
m.showCount(); // outputs 11
console.log(m.count); // outputs 0

The exported functions work as expected. But I'm not clear why m.count isn't also 11.

exports.count = count

Your setting a property count on an object exports to be the value of count. I.e. 0.

Everything is pass by value not pass by reference.

If you were to define count as a getter like such :

Object.defineProperty(exports, "count", {
  get: function() { return count; }
});

Then exports.count would always return the current value of count and thus be 11

Correct me if I am wrong, but numbers are immutable types. When you change the value of count then your reference changes too. So exports.count references to the old count value.

In JavaScript, functions and objects (including arrays) are assigned to variables by reference, and strings and numbers are assigned by value--that is, by making a copy. If var a = 1 and var b = a and b++, a will still equal 1.

On this line:

exports.count = count; // let's also export the count variable itself

you made a by-value copy of the count variable. The setCount(), incCount() and showCount() operations all operate on the count variable inside the closure, so m.count doesn't get touched again. If those variables were operating on this.count, then you'd get the behavior you expect--but you probably don't want to export the count variable anyway.