Category: JavaScript Answers

Sometimes, we want to mock one specific method of a class with Jest and JavaScript.

In this article, we’ll look at how to mock one specific method of a class with Jest and JavaScript.

How to mock one specific method of a class with Jest and JavaScript?

To mock one specific method of a class with Jest and JavaScript, we can use the jest.spyOn method.

For instance, we write

import Person from "./Person";

test("Modify only instance", () => {
  let person = new Person("foo", "bar");
  let spy = jest.spyOn(person, "sayMyName").mockImplementation(() => "Hello");

  expect(person.sayMyName()).toBe("Hello");
  expect(person.bla()).toBe("bla");
  spy.mockRestore();
});

to mock the person.sayMyName method by calling spyOn and mockImplementation in the test.

The mockImplementation callback has the faked implementation of person.sayMyName.

Likewise, we can mock the same method for all tests by writing

import Person from "./Person";

beforeAll(() => {
  jest.spyOn(Person.prototype, "sayMyName").mockImplementation(() => "Hello");
});

afterAll(() => {
  jest.restoreAllMocks();
});

test("Modify class", () => {
  let person = new Person("foo", "bar");
  expect(person.sayMyName()).toBe("Hello");
});

We call jest.spyOn with Person.prototype and 'sayMyName and mockImplementation to do the mocking in the beforeAll callback.

The beforeAll callback runs before any tests are run.

Then we call restoreAllMocks to restore all mocks after all tests are run.

Conclusion

To mock one specific method of a class with Jest and JavaScript, we can use the jest.spyOn method.

Sometimes, we want to fix Phantom.js not waiting for full page load with JavaScript.

In this article, we’ll look at how to fix Phantom.js not waiting for full page load with JavaScript.

How to fix Phantom.js not waiting for full page load with JavaScript?

To fix Phantom.js not waiting for full page load with JavaScript, we can call page.render in the setTimeout callback.

For instance, we write

page.open(address, (status) => {
  if (status !== "success") {
    console.log("Unable to load the address!");
    phantom.exit();
  } else {
    window.setTimeout(() => {
      page.render(output);
      phantom.exit();
    }, 1000);
  }
});

to call page.open with the address URL string to open the page.

And then we call setTimeout with a callback that calls page.render to render the page.

We specify that we wait 1 second before rendering.

Conclusion

To fix Phantom.js not waiting for full page load with JavaScript, we can call page.render in the setTimeout callback.

Sometimes, we want to handle specific errors in JavaScript.

In this article, we’ll look at how to handle specific errors in JavaScript.

How to handle specific errors in JavaScript?

To handle specific errors in JavaScript, we use the instanceof operator.

For instance, we write

try {
  //...
} catch (e) {
  if (e instanceof SpecificError) {
    // ...
  } else {
    throw e;
  }
}

to check if e in the catch block is an instance of the SpecificError constructor with

e instanceof SpecificError

Then we do what we want if that’s true.

Conclusion

To handle specific errors in JavaScript, we use the instanceof operator.

Sometimes, we want to listen to input value change with JavaScript.

In this article, we’ll look at how to listen to input value change with JavaScript.

How to listen to input value change with JavaScript?

To listen to input value change with JavaScript, we can call addEventListener to add the change event listener.

For instance, we write

<input type="text" name="thing" value="" />

to add an input.

Then we write

const doThing = (e) => {
  console.log(e.target.value);
};

document.getElementsByName("thing")[0].addEventListener("change", doThing);

to select the input by its name attribute value with

document.getElementsByName("thing")[0]

Then we call addEventListener to listen to the change event with the doThing function, which is emitted when we change the input value.

In doThing, we get the input value with e.target.value.

Conclusion

To listen to input value change with JavaScript, we can call addEventListener to add the change event listener.

Sometimes, we want to fix cannot set property of undefined with JavaScript.

In this article, we’ll look at how to fix cannot set property of undefined with JavaScript.

How to fix cannot set property of undefined with JavaScript?

To fix cannot set property of undefined with JavaScript, we should make sure we’re setting a property on an object.

For instance, we write

d = {
  [a]: {
    greetings: b,
    data: c,
  },
};

d[a]["greeting"] = "hello";

to assign d to an object.

Then we can set d[a]["greeting"] to 'hello' since d[a] is an object that has the greetings property.

Conclusion

To fix cannot set property of undefined with JavaScript, we should make sure we’re setting a property on an object.