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I'm trying to test my app using Jest and Node.js. What is the correct setup to avoid getting the following error in the terminal when running tests with JestJS?

Cannot read property 'addEventListener' of null

The test for the sum function passes as soon as I ment out adding the event listener in the app.js file. I'm not even sure why is this line as well as the console.log('Not part...') executed by Jest since I only export the sum function.

The contents of my index.html file:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
</head>
<body>

  <button id="button">JavaScript</button>

  <script src="./app.js"></script>
</body>
</html>

The contents of my app.js file:

function sum(a, b) {
  return a + b;
}


console.log('Not part of module.exports but still appearing in terminal, why?');
var button = document.getElementById('button');
button.addEventListener('click', function(e) {
  console.log('button was clicked');
});


module.exports = {
  sum
};

The contents of my app.test.js file:

var { sum } = require('./app');

describe('sum', () => {
  test('adds numbers', () => {
    expect(sum(1, 2)).toBe(3);
  });
});

My package.json:

  "scripts": {
    "test": "jest --coverage",
    "test:watch": "npm run test -- --watch"
  },

I'm trying to test my app using Jest and Node.js. What is the correct setup to avoid getting the following error in the terminal when running tests with JestJS?

Cannot read property 'addEventListener' of null

The test for the sum function passes as soon as I ment out adding the event listener in the app.js file. I'm not even sure why is this line as well as the console.log('Not part...') executed by Jest since I only export the sum function.

The contents of my index.html file:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
</head>
<body>

  <button id="button">JavaScript</button>

  <script src="./app.js"></script>
</body>
</html>

The contents of my app.js file:

function sum(a, b) {
  return a + b;
}


console.log('Not part of module.exports but still appearing in terminal, why?');
var button = document.getElementById('button');
button.addEventListener('click', function(e) {
  console.log('button was clicked');
});


module.exports = {
  sum
};

The contents of my app.test.js file:

var { sum } = require('./app');

describe('sum', () => {
  test('adds numbers', () => {
    expect(sum(1, 2)).toBe(3);
  });
});

My package.json:

  "scripts": {
    "test": "jest --coverage",
    "test:watch": "npm run test -- --watch"
  },
  • javascript
  • node.js
  • testing
  • jestjs
Share Improve this question edited Mar 4, 2017 at 12:36 Piotr Berebecki asked Mar 4, 2017 at 11:26 Piotr BerebeckiPiotr Berebecki 7,4684 gold badges35 silver badges42 bronze badges 1
  • 1 getElementById might execute too soon. Maybe put that code block in window.load = function () { ... }. – trincot Commented Mar 4, 2017 at 14:17
Add a ment  | 

1 Answer 1

Reset to default 9

getElementById might execute before the DOM is loaded. Put that code block in a callback that is executed when the document has been loaded. For instance:

document.addEventListener('DOMContentLoaded', function () {
    console.log('Not part of module.exports but still appearing in terminal, why?');
    var button = document.getElementById('button');
    button.addEventListener('click', function(e) {
      console.log('button was clicked');
    });
});

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