Javascript Undefined

foo.js

/**
 * Undefined cannot be assigned using window.undefined = someVal or
 * undefined = someVal but can be changed using function scope.
 * window.undefined is the same as window['undefined']
 *
 * The same example does not work with the null keyword and program throws
 * a syntax error (function (null) {...})(123)
 */
;(function (undefined) {
  // x is undefined
  let x;

  if (x === undefined) {
    // Will NOT run as [undefined !== 123]
    console.log('Foo');
  }

  if (typeof x === 'undefined') {
    // Will RUN
    console.log('Bar');
  }

  console.log(`undefined is ${undefined}`); // -> undefined is 123
})(123);


/**
 * Solution
 * Create a another function scope for your own code
 */
;(function (undefined) {
  
  (function (undefined) {
    'use strict';

    let x;

    if (x === undefined) {
      // Will Run
      console.log('Foo');
    }

    if (typeof x === 'undefined') {
      // Will RUN
      console.log('Bar');
    }

    console.log(`undefined is ${undefined}`) // -> undefined is undefined
  })()

})(123);


/**
 * ES5 best practice
 */
;(function (global, undefined) {
  'use strict';

  // App code
})(window)