As a fellow JavaScript developer, I know you‘re familiar with the love-hate relationship we have with this flexible yet temperamental language. We use it to add interactivity and flair to websites, but it can also introduce headaches when things go wrong.
In my experience debugging and optimizing web apps over the years, I‘ve noticed some error patterns pop up again and again. I wanted to share with you the most frequent causes of JavaScript errors I encounter, along with my best tips for avoiding them. My hope is that you‘ll find this guide helpful as you build and maintain your own JS-driven projects!
DOM-Related Errors
The Document Object Model (DOM) is like the foundation of a house – it underpins everything we see and interact with on a webpage. So naturally, the DOM is involved in a large portion of JavaScript errors.
In a 2022 survey of over 1,500 web developers, DOM manipulation errors were ranked as the #1 cause of difficult-to-solve JS bugs. I‘ve been there too – trying to grab an element that hasn‘t loaded yet or modifying the wrong node because of scope issues. No fun!
One of the most frequent DOM-related pitfalls is attempting to reference an element before it exists on the page:
// index.html
<script>
const header = document.getElementById(‘page-header‘);
header.textContent = ‘Hello world!‘;
</script>
<h1 id="page-header"></h1>Since scripts load synchronously by default, our code executes before the <h1> tag has rendered. We can fix this by waiting for the DOMContentLoaded event:
document.addEventListener(‘DOMContentLoaded‘, () => {
//DOM is ready!
const header = document.getElementById(‘page-header‘);
header.textContent = ‘Hello world!‘;
});Scope issues are another common source of DOM-related bugs. Accidentally overriding global variables or referencing incorrect contexts for this leads to hair-pulling moments debugging in the browser.
Proper use of closures and immediately invoked function expressions (IIFEs) helps avoid polluting the global namespace. And tools like TypeScript can catch confusing scope references during development.
The DOM powers so much of what we do as web developers. Mastering it is a journey, but preventing common errors gives us the best experience building sites and apps with JavaScript.
Syntax Errors
Nothing throws off your coding groove like seeing a nasty red syntax error on the screen. While seasoned developers make them less often, little typos and mismatches still slip in.
The JavaScript engine parses our code to interpret what we‘ve written. When it encounters invalid syntax that doesn‘t conform to language rules, it throws its hands up and ceases execution.
Some examples I see frequently:
- Mismatched or unclosed brackets
- Missing commas in object literals
- Improper semicolon usage
Little things, but they can have an outsized impact on our programs.
To avoid syntax mishaps, I recommend thoroughly learning JavaScript grammar and syntax conventions. Services like JSFiddle are great sandboxes to experiment and gain more comfort with how the language works.
Linting tools like ESLint also help catch syntax issues automatically during development. I‘m a big proponent of linting – it saves me countless headaches identifying problems before they make it to production.
And keep an eye out for common parsing gotchas like automatic semicolon insertion (ASI). The JS engine tries to fix some things under the hood, but it‘s better to just be explicit in our syntax.
With practice and the right tools, syntax errors go from frustrating to minor annoyances we can swat away.
Null vs. Undefined
While they seem similar on the surface, the null and undefined values in JavaScript have crucial differences under the hood. Mixing them up can lead to some confusing bugs.
null represents an intentional absence of a value:
let guest = null; //guest is unknown right nowThough counterintuitively, null is its own type: an object!
console.log(typeof null); //"object" On the other hand, undefined indicates an uninitialized variable or missing property:
let age;
console.log(age); //undefinedAnd it is a primitive data type:
console.log(typeof undefined); //"undefined"You can see how these two can surprise us – they seem interchangeable but JavaScript treats them differently. Especially when comparing:
null == undefined //true
null === undefined //falseMy advice is to pick one and stick with it for representing unknown values (I prefer null). And be very wary when comparing – use strict equality operators like === to avoid coercion.
Initializing variables properly goes a long way too. I‘ve debugged my fair share of undefined headaches that were due to dependencies not being defined before use.
Undefined Methods and Properties
If you‘ve done much JavaScript programming, you‘ve probably seen this error:
TypeError: Cannot read property ‘foo‘ of undefined
We try calling a method or accessing a property that doesn‘t exist – boom, exception thrown.
Typos when referencing properties are common culprits. We expect autocompletion to save us, but it doesn‘t catch a presnet that should be present.
Inheritance and scope issues also lead to these frustrating bugs. Is that method on the prototype like we think? Did we overwrite a variable‘s value unexpectedly?
I avoid headaches by using TypeScript‘s compiler for catching nonexistent properties during development. Runtime checks with typeof also prevent undefined value exceptions:
// only access property if defined
if (typeof someObj.prop !== ‘undefined‘) {
let propValue = someObj.prop;
}
// only call method if it‘s a function
if (typeof someObj.method === ‘function‘) {
someObj.method();
}A ounce of prevention is worth a pound of debugging here.
Return Statement Bugs
Return statements are a bread-and-butter part of defining function logic. But like any tool, they can introduce issues when used improperly.
A common mistake is accidentally returning prematurely without specifying a value:
function calculateTotal(subtotal, tax) {
return; // oh no, we forgot to return something!
let total = subtotal + tax;
}Or we intend to return values conditionally, but forget to cover all cases:
function parseInput(val) {
if (val) {
return val;
}
// forgot to handle falsy case
}Linters help catch some return statement errors automatically. I also advocate unit testing key functions – mistakes in return values become obvious when tests start failing.
The best medicine is taking time to think through all possible code pathways and return cases upfront. Easier said than done of course, but conscious refactoring helps us improve over time.
Parting Advice
JavaScript has its quirks and rough edges, but by understanding common causes of errors, we can avoid lots of headaches. Some parting advice:
- Use linting and TypeScript to catch issues early
- Write comprehensive unit tests for critical functions
- Learn language internals like scopes, contexts, and objects
- Refactor code purposefully to incorporate lessons learned
- View errors as opportunities to improve skills
I hope reflecting on these common JS pitfalls is useful to you in building robust applications. We‘ll all keep becoming better developers by learning from mistakes and building good habits. Happy bug hunting!
