Strict mode

Strict mode is used to show more specific errors that JavaScript doesn't show by default.

'use strict'

It is also possible to only use strict mode in a certain scope, this is true for everything nested inside the scope, for example:

function scriptMode() {
  'use strict';
  console.log("This message is in strict mode!");
  function nestedScriptMode() {
    console.log("This is also in strict mode!");
  }
}
console.log("This message is not in strict mode");

If you make a module, it is automatically set to strict mode.