Donald Knuth has been famously quoted as saying "premature optimization is the root of all evil". That's because he'd never tried JavaScript...

JavaScript has been the most popular programming language in the Stack Overflow Developer Survey for 12 years in a row (as of 2024). Why is this? Is it because it's well designed? Or because it's fast? Well... no. JavaScript is one of the most poorly designed programming languages, and is individually responsible for wasting an incomprehensible number of human hours producing terrible software which has ultimately trained the average user to think their computer is way slower than it actually is.

Why is JavaScript Bad?

I can give a long list of features and design decisions of JavaScript which are absolutely terrible, but I think the horror of JS is best shown by giving examples of actual programs and algorithms, to show where these things actually show up.

== Doesn't Check Equality

Let's start with a simple one. a == b can be true, with a and b being obviously different things. Here are some examples of equality checks, all of which are true:

[] == false
'' == false
'2' == 2
1 == true
0 == false
'0' == false
null == undefined

Now it can be argued whether pointer equality or deep equality should be used in a programming language, however at least in those cases the rules are consistent and clear. The JS standard which describes how == should work is an absolutely absurd list of 14 rules with sub-rules, which in turn reference complicated type conversion functions. This is a classic example of the fact that because the way that JavaScript interprets so many simple, single lines of code is so deeply unintuitive, you end up spending more time debugging your understanding of the language than you do actually writing code.

If you've never programmed in JavaScript before this will seem crazy. But so many people, even those who hate JS, have had to program in it enough that the Stockholm syndrome starts to sync in and they forget just how moronic of a design decision it is to have the equality operator not check equality.

Function Argument Permissiveness

The combination of first class functions with an extremely dynamic type system lead to some interesting points of frustration when programming. Let's say you write a simple function which adds the first n natural numbers together:

let naturalNumberSum = function(n) {
    let sum = 0;

    for (let i = 0; i <= n; i++) {
        sum += i;
    }
    return sum;
};

but very quickly realise that you need to generalise this function to accept a lower bound other than zero, for example to calculate the sum of all natural numbers between 10 and 20 inclusive, so you make the appropriate changes:

let naturalNumberSum = function(lower, upper) {
    let sum = 0;

    for (let i = lower; i <= upper; i++) {
        sum += i;
    }
    return sum;
};

Great, now you can use the function to calculate the desired value:

naturalNumberSum(10, 20)

Except what about all the cases where you've previously used the function for its old purpose. In any language with proper static type checking, each of these instances would be a compiler error which can be systematically fixed, changing naturalNumberSum(x) to naturalNumberSum(0, x).

In JavaScript though, the function gets called just fine with what was previously bound to x as lower, and upper as undefined. So we have to deal with some strange runtime error because upper is undefined right? Well no, JavaScript's permissiveness of undefined values means the function actually runs just fine, it just gives us the wrong output:

> naturalNumberSum(10)
0

thus leaving a bunch of nefarious bugs which we have to carefully and diligently hunted down.

A more experienced JavaScript programmer would probably solve this problem by having special checks for when upper is undefined, and doing a sort of runtime polymorphism as follows:

let naturalNumberSum = function(lower, upper) {

    if (upper == undefined) {
        upper = lower;
        lower = 0;
    }

    let sum = 0;

    for (let i = lower; i <= upper; i++) {
        sum += i;
    }
    return sum;
};

Not only does this make our code much harder to read, we actually now need to worry about the implications of our upper == undefined statement evaluating to true in undesired cases, such as if upper is null.

So when any other language would prevent this problem at compile time, and potentially even allow the statically checked overloading, JavaScript forces us to write harder to read and more error prone code.

Approach to Incorrect Code

JavaScript has decided that rather than deal with errors and help the programmer correct them, it is better to ignore them and pretend they don't exist. This is through turning what would be compilation errors in other languages into runtime or logic errors.

Let's say you have an object which stores a boolean field on it:

let person = {
    firstName: "John",
    lastName: "Doe",
    age: 52,
    deceased: true
};

and at some point in your program you wish to do some logic depending on that boolean field:

if (person.deceaced)
{
    // do something
}
else
{
    // do something else
}

Note the typo in the condition of the if statement. What happens when I make this mistake in C, C++, Rust, Java, Swift or Go for example? The compiler tells me. It explicitly states that the field does not exist. What do Python and R do? Well you have to wait until the interpreter hits that line to identify the error, which is not quite as good, but at least you still find out. What does JavaScript do, well because it didn't statically check the types ahead of time it just treats person.deceaced as undefined. This means we are evaluating if (undefined), and because undefined is falsy (i.e. it coerces to false), the else case runs, even though our field is true. So here, a typo causes not a compiler error, not even a runtime error, but a logic error.

Now this example may seem simple to fix, but it's not when you've got a codebase of thousands of lines, or you're using a field on an object returned from a library, and you haven't quite capitalised the name correctly because it uses a different styleguide. This means when programming you have to constantly check these minor things.

And no, powerful linters in editors are not an acceptable solution to this problem. The web developer mentality of just adding a band aid fix to everything which is bad because it was designed poorly is what causes so many of these issues to begin with.

This is one of many examples of JavaScript's allergy to compile and runtime errors, along with its aggressive type coercion and permissiveness around undefined values particularly with function arguments, which dramatically slow the software development process and introduce countless logic errors.

More Examples of JavaScript Being Terrible

If you have yet to be convinced that JavaScript is poorly designed, here is a collection of examples of showing off the many reasons why JavaScript is terrible:

• Less Than or Equal to Not Less Than or Equal to
• Bugs From Automatic Semicolon Insertion
• Less Than or Equal to Is Not a Total Order
• Negation Equals Itself
• == Is Not an Equivalence Relation
• Arithmetic Inconsistency
• Tautology Is False
• String Is Not a String?
• What on Earth Is this?

The Broader Issue

JavaScript is not just bad because it's a poorly designed language, it represents a direction the software industry as a whole is taking, moving towards software with so many layers of abstraction that make it impossible to reasonably predict the performance of a program. So often these new fancy tools are created to improve so called "developer productivity", but end up reducing software quality. Think about how many UI frameworks there are for JavaScript, some built on top of others. Have any of them really solved the core problem?

Most web developers simply cannot conceive of how fast computers can be, because their language is slow, their tooling is slow, their industry rarely makes performance a requirement, and any attempt to make their code fast may result in them being shot in the foot because of hidden behaviour of the web rendering engine.

The worst thing though is that most people don't see this as a problem.

Most people don't seem to put two and two together when game developers can reliably render 60 different high definition images a second whilst tracking the state of thousands of objects, yet discord takes 5 seconds to start up on a computer with a 16 core 12th generation i5 and 32 gigabytes of ram. Because rather than just draw pixels on the screen, when your computer runs a JavaScript app like Discord, it's reading the source code while running it, in this hugely complicated browser engine, while manipulating this additional UI abstraction called the DOM. All while making sure to render things correctly given a separate text based document call CSS.

And this is not just a user experience issue. Software written in JavaScript regularly executes thousands of times the number of CPU instructions necessary to complete the relevant task, which equates to thousands of times the power consumption, and thus green house gas emissions. With all the software in the world written in JS, the greenhouse gasses emitted by computers running way more instructions that necessary becomes seriously non-trivial. So yes, I am going to be that guy who makes the "JS is destroying the planet" argument.

The 7 Step Plan to End JS

So, I now present a 7 step plan to put an end to JavaScript.

Further Reading

I, the creator of this site, am not the only one who hates JavaScript, and this is not the first piece of online material dedicated to the case against JS, so here are some other great blog posts, articles and videos about why JavaScript is bad, how to live without out, and the problems with the current state of web development:

• The Top 10 Things Wrong with JavaScript
• Why JavaScript Makes Bad Developers
• I Turned Off JavaScript for a Whole Week and It Was Glorious
• Why I don't suggest JavaScript as a first programming language
• The Problem with the Web
• Awesome No JS Web Frameworks
• The Dark Side of JavaScript
• A Collection of JavaScript Gotchas
• Why Does JavaScript Suck?
• James Mickens On JavaScript
• YourLanguageSucks: JavaScript
• Stop Being Cute And Clever