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Common JavaScript Bugs

posted on: 08-jan-2019

JavaScript is the most fragile part of the web stack, and there’s a million different things that can go wrong.

But, in my experience there’s usually a handful of common mistakes that happen more than others. Let’s look at what they are.

Using a single = instead of a comparison operator

One super easy thing to do is use = instead of == or === in a for statement.

const sandwich = document.querySelector('.sandwich');

// Wrong
if ( = 'tuna') {
	// Do something...

// Right
if ( === 'tuna') {
	// Do something...

Comparing mismatched types

There are two types of “equals” comparison operators in JavaScript: equals (==) and strict equals (===).

Equals uses something called type coercion to compare things. If one item is a number and the other is a string, it will force them to both be strings to do it’s comparison.

// This will evaluate as "true"
if (5 == '5') {
	// Do something...

Strict equals compares items, well, strictly, and requires them to be the same type, not just the same value, to be true.

// This evaluates as "false"
if (5 === '5') {
	// Do something...

It’s really easy to compare a string to a number when getting values from the inputs or the DOM, and get a false comparison when you expect true.

Missing brackets and parentheses

It’s easy to miss a closing brackets and parentheses, especially when dealing with nested statements.

// This is missing a closing bracket on the second if statement
if (sandwich === 'tuna') {
	if (bread === 'wheat') {
		alert('What a yummy sandwich!');

// This is missing a closing parentheses
// Which isn't surprising because this is an absurd example!
if (100 - ((10 - 5) / 2 * (11 + 3) === 42) {
	// Do something...

Unescaped quotes in a string

If you include quotes in a string, they need to be escaped with a backslash (\) or they’ll close out the string.

This applies to quotes that are the same type as the wrapping quotes. So if you use single quotes for your strings, you need to escape any single quotes in the text. If you use double quotes, you need to escape double quotes.

// This will cause an error
const str1 = "She said, "I'm going to get a sandwich." I hope it's tuna!";

// This will not. The backslashes won't show up, either
const str1 = "She said, \"I'm going to get a sandwich.\" I hope it's tuna!";

Typos and capitalization

Another common cause of bugs is mistyped variables.

Sometimes they’re misspelled. Variable names are also case-sensitive, though, so if you used all lowercase on a camelCase variable (or vice-versa), you’ll get an error.

var mySandwich = 'tuna';

// Wrong
if (mySandiwch === 'tuna') {
	// Do something....

// Also wrong
if (mysandwich === 'tuna') {
	// Do something...

// Right
if (mySandwich === 'tuna') {
	// Do something...

Trying to run methods on things that don’t exist.

This is really common when using DOM selectors like querySelector(). If you try to use a method like classList() or access a property like .id on an element that doesn’t exist, you’ll get an error.

const elem = document.querySelector('#does-not-exist');

// Throws error: "Uncaught TypeError: Cannot read property 'id' of null"
const id =;

// Throws error:  "Uncaught TypeError: Cannot read property 'closest' of null"
if (elem.closest('tuna-sandwich')) {
	// Do something...