NodeJS全局变量不可重新分配

Question

I am trying to unit test code that uses JavaScript library that is set a global variable if it does not exist. The pattern the library is using is:

var GLOBAL_VAR = GLOBAL_VAR || {}

This works in browser world, but when I execute the code in NodeJS, it does not work. The problem comes down to this:

var myGlobal = 'CORRECT';
console.log('Prints CORRECT', myGlobal || 'WRONG');
(function () {
  // Why does this print WRONG?
  var myGlobal = myGlobal || 'WRONG';
  console.log('Prints WRONG', myGlobal);
}).call(this);

(function () {
  console.log('Prints CORRECT', myGlobal || 'WRONG');
}).call(this);

Why is the first function printing WRONG while the second function prints CORRECT?

Answer 1

You are declaring a local variable myGlobal inside the first anonymous function. This shadows the global variable.

Then, in the anonymous function, you declare:

var myGlobal = myGlobal || 'WRONG';
//             ^
//             |  this local variable is undefined here, as
//                the global is not accessible with this name

That is why myGlobal (the local variable) gets the value 'WRONG' .

The solution is to rename the confusingly named local variable myGlobal in the anonymous function to something that does not shadow the global variable.

Note that you would not have this problem if you used let as let does not allow you to use the variable as a value in it's own declaration:

let x = x || 'WRONG'; // should produce an error and leave x undefined.