Common errors in Javascript and how it's solved in ELM

August 15, 2018

One of my favorite things about ELM is the compiler, it will catch errors before they could make their way to production. Let’s have a look at the most common Javascript errors and see how ELM makes sure they don’t bother you.

Uncaught TypeError: Cannot read property

Imagine you wrote this code in Javascript:

function legal(age) {
  if (age > 18) {
    return {
      isLegal: true,
      isTooOld: age < 90

You would call const l = legal(21) and then try to access l.isTooOld, everything works, you create a PR, wait for peer review, merge and deploy to production.

One hour later, you receive an alert from your reporting service saying Uncaught TypeError: Cannot read property 'isTooOld' of undefined, so you dig into it, debug it, create a new PR, wait for peer review, and deploy to production. In the end, you waisted 1-2 hours. The example is trivial but this issue happens every day to millions of innocent programmers.

Now let’s see in ELM:

legal: Int -> {isLegal: Bool, isTooOld: Bool}

In ELM a function can only return one data shape in comparison to the example in Javascript above that can return an object or undefined.

You might say “what if I actually want to return nothing”. If a function needs to return nothing, then you would return a Maybe a. Eg: Let’s say I have a list of records (objects) and I want to create a function to find a specific record from that list. If the record is found it would be returned otherwise no data would be returned. Let’s see our function’s signature:

findById: Int -> List record -> Maybe record

By this way, whenever another function calls findById, it must handle the possibility that it doesn’t return data.

case findById 1 users of
    Nothing ->
        do something
    Just user ->
        do something else

Another way to produce an error? Maybe create an object and try to access a key that doesn’t exist:

a = { b = { c = 2 } }
`a` does not have a field named `foo`.

The type of `a` is:

    { b : { c : number } }

Which does not contain a field named `foo`.

Hint: The record fields do not match up. One has b. The other has foo.

The compiler caught it so your clients don’t have to!

Uncaught TypeError: xxx is not a function

Imagine you need to remove a function in the code, you will remove all calls that you can find to this function then deploy. A few hours later, notification pops up again from your error reporting service saying Uncaught TypeError: evaluate is not a function.

In ELM if you remove a function’s definition but that function is still called somewhere, then the compiler will tell you about it. Eg:

reduce fn acc v =
main =
    foo identity [1,2,3]
foo fn v =
    reduce fn 0 v
Line 5, Column 5
Cannot find variable `reduce`

Uncaught RangeError

This error happens when your call stack size reached its limit. Most common case is a recursive function that never ends. This could happen if the recursive function is poorly coded and doesnt have an exit branch or if the parameters are not what is expected.

Elm on the other hand can detect some bad recursion definitions:

ones = 1 :: ones
`ones` is defined directly in terms of itself, causing an infinite loop.

Maybe you are trying to mutate a variable? Elm does not have mutation, so when I
see `ones` defined in terms of `ones`, I treat it as a recursive definition. Try
giving the new value a new name!

Maybe you DO want a recursive value? To define `ones` we need to know what
`ones` is, so let’s expand it. Wait, but now we need to know what `ones` is, so
let’s expand it... This will keep going infinitely!

To really learn what is going on and how to fix it, check out:

The following would still break at runtime. Fortunately it’s a known issue and the next version of ELM (0.19) might catch this as well:

loop x = loop x

See this issue for more details on bad recursion catching strategy in ELM compiler.

Final words

I omitted a lot of Javascript common errors because they were either similar but on different browsers or were too specific. ELM has virtually no runtime error, thanks to the compiler and the well thought language. That being said, if you really want to make your ELM app to crash at runtime just check the top 6 ways to make your elm app crash at runtime by eeue56.