What's the benefit of using "function() 'use strict'" in every file? [duplicate]

Question

I'm refactoring an old AngularJS 1.3 project. One of the things I noticed is that the person who made this started every single file of AngularJS code with:

(function () {
'use strict';

  angular.module('app').factory('Employees', ['$http', function($http) {
    // angular code removed
  }]);

})();

Does using function() 'use strict' in every single file any benefit to the code? To me it feels like a waste of 3 lines in every single file. Is there a standard / best practice for this?

Answer 1

Using use strict helps you to prevent problems with closures and variable scopes.

For example, if you accidentaly set a global variable - in this case by forgetting to add var keyword in the for loop, use strict mode will catch it and sanitize.

(function(){
  for (i = 0; i < 5; i++){
  }
  console.log(i);
})();

.as-console-wrapper { max-height: 100% !important; top: 0; }

(function(){
  'use strict';
  for (i = 0; i < 5; i++){
  }
  console.log(i);
})();

.as-console-wrapper { max-height: 100% !important; top: 0; }