Author: John Au-Yeung

Web developer specializing in React, Vue, and front end development.

Categories
React Answers

How to detect when user scrolls to bottom of div with React?

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Sometimes, we want to detect when user scrolls to bottom of div with React.

In this article, we’ll look at how to detect when user scrolls to bottom of div with React.

How to detect when user scrolls to bottom of div with React?

To detect when user scrolls to bottom of div with React, we check if the sum of the scrollTop and clientHeight of the scroll container is the same as its scrollHeight.

For instance, we write

import React, { useRef, useEffect } from "react";

const MyListComponent = () => {
  const listInnerRef = useRef();

  const onScroll = () => {
    if (listInnerRef.current) {
      const { scrollTop, scrollHeight, clientHeight } = listInnerRef.current;
      if (scrollTop + clientHeight === scrollHeight) {
        // ...
        console.log("Reached bottom");
      }
    }
  };

  return (
    <div className="list">
      <div className="list-inner" onScroll={onScroll} ref={listInnerRef}>
        {/* List items */}
      </div>
    </div>
  );
};

to call onScroll when we scroll by setting the onScroll prop of the div to onScroll.

Then we assign a listInnerRef to the div.

In onScroll, we check if the sum of the scrollTop and clientHeight of the scroll container is the same as its scrollHeight.

If it is, then the bottom of the element is reached.

Conclusion

To detect when user scrolls to bottom of div with React, we check if the sum of the scrollTop and clientHeight of the scroll container is the same as its scrollHeight.

Categories
JavaScript Answers

How to check if a number is prime in JavaScript?

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Sometimes, we want to check if a number is prime in JavaScript.

In this article, we’ll look at how to check if a number is prime in JavaScript.

How to check if a number is prime in JavaScript?

To check if a number is prime in JavaScript, we can use a loop.

For instance, we write

const isPrime = (num) => {
  for (let i = 2, s = Math.sqrt(num); i <= s; i++) {
    if (num % i === 0) {
      return false;
    }
  }
  return num > 1;
};

to define the isPrime function.

In it, we loop from 2 to the square root of num.

In the loop body, we check if num can be evenly divisible by i with num % i === 0.

And if it’s true, we return false since it’s not prime.

If it’s evenly divisible by nothing, we return true.

Conclusion

To check if a number is prime in JavaScript, we can use a loop.

Categories
JavaScript Answers

How to check for IP addresses with JavaScript?

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Sometimes, we want to check for IP addresses with JavaScript.

In this article, we’ll look at how to check for IP addresses with JavaScript.

How to check for IP addresses with JavaScript?

To check for IP addresses with JavaScript, we use the is-ip module.

We install it with

npm i is-ip

Then we use it by writing

const isIp = require("is-ip");

const isItIp = isIp("192.168.0.1");

to call isIp with a string that we want to check if it’s a string with an IP address.

Conclusion

To check for IP addresses with JavaScript, we use the is-ip module.

Categories
JavaScript Answers

How to get the max value of scrollLeft with JavaScript?

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Sometimes, we want to get the max value of scrollLeft with JavaScript.

In this article, we’ll look at how to get the max value of scrollLeft with JavaScript.

How to get the max value of scrollLeft with JavaScript?

To get the max value of scrollLeft with JavaScript, we can use the scrollWidth and clientWidth propeties.

For instance, we write

const maxScrollLeft = element.scrollWidth - element.clientWidth;

to subtract the element‘s scrollWidth value by its clientWidth value.

Then we get the max value of scrollLeft.

Conclusion

To get the max value of scrollLeft with JavaScript, we can use the scrollWidth and clientWidth propeties.

Categories
JavaScript Answers

How to add an img element to a div with JavaScript?

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Sometimes, we want to add an img element to a div with JavaScript.

In this article, we’ll look at how to add an img element to a div with JavaScript.

How to add an img element to a div with JavaScript?

To add an img element to a div with JavaScript, we use the appendChild method.

For instance, we write

document.getElementById("placehere").appendChild(elem);

to get the element with ID placehere with getElementById.

Then we call appendChild to add the elem img element as the last child of the element with ID placehere.

Conclusion

To add an img element to a div with JavaScript, we use the appendChild method.