CODEWITHSUNDEEP
javascript
Wertu
Wertu

Reputation: 137

Is 'use strict' implied when ES modules is used?

I was learning JS and came across 'use strict'. Then, here Should I 'use strict' for every single javascript function I write? @Bergi says that "you only should put 'use strict' in the module scope - once per file - so that it is inherited by all your functions". Is it true that by "it is inherited by all your functions" means each function in a certain module uses 'use strict' inside of itself?

Upvotes: 9

Views: 2284

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370779

Is it true that by "it is inherited by all your functions" means each function in a certain module uses 'use strict' inside of itself?

A function will run in strict mode if:

(1) The function is inside an ES6 module (these all run in strict mode regardless), or

(2) There is a proper 'use strict' directive above the function definition, lexically.

In addition to a few less common instances, such as inside a constructor.

The way 'use strict' is inherited lexically works the same way variables can be referenced, and how variable scope works - if any outer block of a function is strict, then the inner function is strict as well. So, for example, with the following code:

'use strict';
function foo() {
  function bar() {
    function baz() {
    }
  }
}

No matter what other code you put inside foo, bar, and baz, all of those functions, no matter how they're run, will run in strict mode. If one of these function is called from a source which isn't running in strict mode, the called function will still run in strict mode. For example:

<script>
'use strict';
function foo() {
  return function bar() {
    return function baz() {
      console.log('baz running. strict:', this === undefined);
    }
  }
}
foo()()();
</script>
<script>
// Non-strict script calling strict function:
foo()()();
</script>

Upvotes: 8

Related Questions