Category: JavaScript Answers

Sometimes, we want to clear timeouts and intervals with React hooks.

In this article, we’ll look at how to clear timeouts and intervals with React hooks.

How to clear timeouts and intervals with React hooks?

To clear timeouts and intervals with React hooks, we call clearTimeout in the callback that we return in the useEffect hook callback.

For instance, we write

const Comp = () => {
  //...
  React.useEffect(() => {
    const timeoutID = window.setTimeout(() => {
      //...
    }, 1000);

    return () => window.clearTimeout(timeoutID);
  }, []);

  //...
};

to call useEffect with a callback that calls setTimout to run its callback with a 1 second delay.

Then we return a function that calls clearTimeout with the timeoutId returned by setInterval to clear the timer when the component unmounts.

Conclusion

To clear timeouts and intervals with React hooks, we call clearTimeout in the callback that we return in the useEffect hook callback.

Sometimes, we want to automatically set the focus to a textbox when a web page loads with JavaScript.

In this article, we’ll look at how to automatically set the focus to a textbox when a web page loads with JavaScript.

How to automatically set the focus to a textbox when a web page loads with JavaScript?

To automatically set the focus to a textbox when a web page loads with JavaScript, we can call the focus method on the text box in the window.onload handler.

For instance, we write

window.onload = () => {
  document.getElementById("Box1").focus();
};

to set window.onload to a function that runs when all the elements on the page are loaded.

In it, we get the text box with

document.getElementById("Box1")

And then we call focus on it.

Conclusion

To automatically set the focus to a textbox when a web page loads with JavaScript, we can call the focus method on the text box in the window.onload handler.

Sometimes, we want to check variable equality against a list of values with JavaScript.

In this article, we’ll look at how to check variable equality against a list of values with JavaScript.

How to check variable equality against a list of values with JavaScript?

To check variable equality against a list of values with JavaScript, we can use the array includes method.

For instance, we write

if ([1, 47, 28].includes(foo)) {
  // ...
}

to call [1, 47, 28].includes with foo to check if foo is one of 1, 47 or 28.

Conclusion

To check variable equality against a list of values with JavaScript, we can use the array includes method.

Sometimes, we want to convert JavaScript object with numeric keys into array.

In this article, we’ll look at how to convert JavaScript object with numeric keys into array.

How to convert JavaScript object with numeric keys into array?

To convert JavaScript object with numeric keys into array, we can use the array map method with Object.keys.

For instance, we write

const arr = Object.keys(obj).map((k) => {
  return obj[k];
});

to call Object.keys with obj to return the an array of keys in obj.

Then we call map with a callback to return the value of property k in obj.

Conclusion

To convert JavaScript object with numeric keys into array, we can use the array map method with Object.keys.

Sometimes, we want to use Jasmine to spy on a function without an object.

In this article, we’ll look at how to use Jasmine to spy on a function without an object.

How to use Jasmine to spy on a function without an object?

To use Jasmine to spy on a function without an object, we can use the spyOn function.

For instance, we write

import * as FooFunctions from "../foo_functions";

x = FooFunctions.foo(y);

to call foo in FooFunctions in the test file.

Then in the test, we write

spyOn(FooFunctions, "foo").and.callFake(() => {
  //...
});
// ...
expect(FooFunctions.foo).toHaveBeenCalled();

to call spyOn with the FooFunctions module and the 'foo' function name string.

And we call callFake with a function that we mock foo with.

And then we call expect to check if foo is called with toHaveBeenCalled.

Conclusion

To use Jasmine to spy on a function without an object, we can use the spyOn function.