JavaScript Tips

JavaScript Tips — Errors, Auto-Resizing Text Area, and More

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Like any kind of apps, there are difficult issues to solve when we write JavaScript apps.

In this article, we’ll look at some solutions to common JavaScript problems.

Replace All Commas in a String

We can replace all commas in a string with the replace method.

For instance, we can write:

const str = 'foo,bar,baz';
const newStr = str.replace(/,/g, '-');

Then newStr is “foo-bar-baz” .

Good Way to Extend Error in JavaScript?

We can extend the Error class with the extends keyword.

For instance, we can write:

class SpecialError extends Error {
  constructor(message) {
    super(message); = 'special error';

We created a SpecialError class which inherits from the Error class.

We can set our own name to distinguish it from the Error instance.

Convert String with Commas to Array

We can convert strings with commas to an array with the split method.

For instance, we can write;

const string = "1,2,3";
const array = string.split(",");

We have a string that we call split on with a comma to split the string into an array.

Add or Update a Query String Parameter

To add or update a query string parameter, we can create a URLSearchParams instance with the query string.

Then we can call set on it with the key and value to set the parameter.

For instance, we can write:

const searchParams = new URLSearchParams(
searchParams.set("foo", "baz");
const newURL = `${window.location.pathname}?${searchParams.toString()}`;
history.pushState(null, '', newURL);

We get the query string in the browser search bar with .

Then we call set with the key and value of it.

Next, we get the query string by calling toString on the URLSearchParams instance.

And we combine the pathname and query string together.

Then we call pushState to with the new URL to reload the page with the new query string.

Open a URL in a New Window Rather than a Tab

To always open a URL in a new window rather than a tab, we can pass in a 3rd argument to with the width and height of the window.

For instance, we can write:, '_blank', "height=200,width=200");

The 3rd argument has the specifications of the window.

Check if a Variable is a Number

We can check if a variable is a number by using the isNaN and isFinite functions.

For instance, we can write:

!isNaN(parseFloat(num)) && isFinite(num);

We use parseFloat to try to parse num to a float.

Then we can check the returned value with isNaN .

We also pass num to isFinite to check that.

If both expressions are true then we know we got a finite number.

Creating a Text Area with Auto-Resize

To create a text area with auto-resize, we can listen to the input event.

Then we can set the height of the text area in the input event listener to the scrollHeight plus 12 pixels.

scrollHeight gets the full height of the text.

To do that we can write:


to create the text area.

Then we can write:

function resizeTextarea(ev) { = 'auto'; = `${this.scrollHeight}px`;
const textArea = document.querySelector('textarea');
textArea.addEventListener('keyup', resizeTextarea);

'auto' resets the height to the original.

Then we set the height to the scrollheight to make it the height of the text.

Why Shouldn’t we use document.write?

We shouldn’t use document.write to render HTML on the screen.

It doesn’t work with XHTML.

It’s run after the page finishes loading and overwrites the page.

Also, it runs where it’s encountered.

It’s doesn’t write content like we do with DOM manipulating and can easily create bugs since we’re writing string’s content onto a page.

Therefore, we shouldn’t use document.write to write content onto the screen.

Check if the URL has a Given String

We can use the indexOf method on window.location.href to check if a URL contains a given string.

For instance, we can write:

window.location.href.indexOf("foo") > -1

to check if 'foo' is in the current page’s URL.

If the returned value is -1, that means it’s not in the URL.

Otherwise, it’s in the URL.


We can use the URLSearchParams constructor to parse and manipulate query strings. can open a new window.

The Error constructor can be extended with a child class.

By John Au-Yeung

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

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