Category: JavaScript

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JavaScript Answers

How to reset or clear a spy in Jest and JavaScript?

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Sometimes, we want to reset or clear a spy in Jest and JavaScript.

In this article, we’ll look at how to reset or clear a spy in Jest and JavaScript.

How to reset or clear a spy in Jest and JavaScript?

To reset or clear a spy in Jest and JavaScript, we call the jest.clearAllMocks method.

For instance, we write

afterEach(() => {
  jest.clearAllMocks();
});

to call jest.clearAllMocks in the afterEach callback to clear all spies after each test is run.

Conclusion

To reset or clear a spy in Jest and JavaScript, we call the jest.clearAllMocks method.

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React Answers

How to use setInterval in a React app?

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Sometimes, we want to use setInterval in a React app.

In this article, we’ll look at how to use setInterval in a React app.

How to use setInterval in a React app?

To use setInterval in a React app, we call setInterval in the useEffect hook callback.

For instance, we write

import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";

const Clock = () => {
  const [currentCount, setCount] = useState(10);
  const timer = () => setCount(currentCount - 1);

  useEffect(() => {
    if (currentCount <= 0) {
      return;
    }
    const id = setInterval(timer, 1000);
    return () => clearInterval(id);
  }, [currentCount]);

  return <div>{currentCount}</div>;
};

const App = () => <Clock />;

to call useEffect with a callback that calls setInterval to run the timer function every 1000 milliseconds.

We return a function that calls clearInterval with the timer id to clear the timer when currentCount changes.

Conclusion

To use setInterval in a React app, we call setInterval in the useEffect hook callback.

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JavaScript Answers

How to set background color of HTML element using CSS properties in JavaScript?

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Sometimes, we want to set background color of HTML element using CSS properties in JavaScript.

In this article, we’ll look at how to set background color of HTML element using CSS properties in JavaScript.

How to set background color of HTML element using CSS properties in JavaScript?

To set background color of HTML element using CSS properties in JavaScript, we can set the backgroundColor style.

For instance, we write

const setColor = (element, color) => {
  element.style.backgroundColor = color;
};

const el = document.getElementById("elementId");
setColor(el, "green");

to create the setColor function.

In it, we set the lement.style.backgroundColor property to color to set the background color to color.

Then we select the element we want to set the background color for with getElementById.

And we call setColor with it and 'green' to set its background color to green.

Conclusion

To set background color of HTML element using CSS properties in JavaScript, we can set the backgroundColor style.

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JavaScript Answers

How to create an accurate timer in JavaScript?

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Sometimes, we want to create an accurate timer in JavaScript.

In this article, we’ll look at how to create an accurate timer in JavaScript.

How to create an accurate timer in JavaScript?

To create an accurate timer in JavaScript, we should get the time from the Date object.

For instance, we write

setInterval(() => {
  console.log(new Date().toUTCString());
}, 1000);

to call setInterval with a callback that gets the latest datetime with the Date constructor.

And we call toUTCString to return a human readable datetime string.

1000 is the 2nd argument so the callback runs about every 1000 milliseconds.

Conclusion

To create an accurate timer in JavaScript, we should get the time from the Date object.

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JavaScript Answers

How to detect Android phone via JavaScript?

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Sometimes, we want to detect Android phone via JavaScript.

In this article, we’ll look at how to detect Android phone via JavaScript.

How to detect Android phone via JavaScript?

To detect Android phone via JavaScript, we can check if the user agent has 'android' in it.

For instance, we write

const ua = navigator.userAgent.toLowerCase();
const isAndroid = ua.indexOf("android") > -1;
if (isAndroid) {
  //...
}

to get the user agent with navigator.userAgent.

Then we convert it to lower case with toLowerCase.

Next we check if 'android' is in the user agent string with indexof.

Conclusion

To detect Android phone via JavaScript, we can check if the user agent has 'android' in it.