I'm getting the following warning when using jshint. Why?

If a strict mode function is executed using function invocation, its 'this' value will be undefined.

function demo() {
    'use strict';

    document.querySelector('#demo').addEventListener('click', test);

    function test() {
        console.log(this);
    }
}

I'm getting the following warning when using jshint. Why?

If a strict mode function is executed using function invocation, its 'this' value will be undefined.

function demo() {
    'use strict';

    document.querySelector('#demo').addEventListener('click', test);

    function test() {
        console.log(this);
    }
}

• javascript
• jshint

Share Improve this question Follow edited Apr 17, 2018 at 16:37 Alvaro asked Apr 17, 2018 at 16:27 AlvaroAlvaro 41.6k3131 gold badges172172 silver badges348348 bronze badges 6

• 1 Because this will indeed be undefined if you invoke test without using some way to provide a definite thisArg. So test() will give you undefined for the this value. – user2437417 Commented Apr 17, 2018 at 16:31
• You'll need to use .call() or .apply() to pass this to the function that's being called trough indirect invocation. I don't know why strict mode does that, but it does. – Phiter Commented Apr 17, 2018 at 16:32
• 1 It'll cause problems if you try to use this as an object (like trying to access properties). Not an issue in that specific function as written. Without strict mode, you're guaranteed an object. – user2437417 Commented Apr 17, 2018 at 16:34
• 1 I guess I'll have to configure jsHint to not fire those warnings? – Alvaro Commented Apr 17, 2018 at 16:36
• 1 Yeah, it's all configurable. jshint./docs/options/#validthis – user2437417 Commented Apr 17, 2018 at 16:36

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3 Answers 3

Sorted by: Reset to default 3

This works for me

function demo() {
  'use strict';

  var that = this; //new line

  document.querySelector('#demo').addEventListener('click', test);

  function test() {
    console.log(that); //print that instead of this
  }
}

Rather than try to suppress the warning, it's better to address the root cause.

Use of this can be confusing, and the value of this can unexpectedly change when refactoring code. Your code will be easier to read and maintain if you explicitly pass parameters.

The parameter passed to the test() callback is the Event object for the click:

function demo() {
   'use strict';
   function test(event) {
      console.log('You clicked on:', event.target.outerHTML); 
      }
   document.querySelector('#demo').addEventListener('click', test);
   }

demo();

Console log output will be something like:
You clicked on: <h1 id="demo">Click Me</h1>

The Event object tells you the target element that the user clicked:
https://developer.mozilla/en-US/docs/Web/API/Event/target

Fiddle with the code:
https://jsfiddle/24epdxbz/2

From yehudakatz:

The ECMAScript 5 spec says that undefined is (almost) always passed, but that the function being called should change its thisValue to the global object when not in strict mode. This allows strict mode callers to avoid breaking existing non-strict-mode libraries.

Sometimes the scope in JS needs that instead of this, according to articles like this Scope in js

thus, you usually will need to use

var that = this;