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Simplified try-catch strategy in DataWeave with the default keyword (instead of try/orElse)


In this post:


The other day I was reading some StackOverflow DataWeave questions (as one does) and I got to this very interesting thread: Why the “default” keyword acts like “try + catch / orElse” in some cases. I wanted to take a moment to create an article about my own explanation of what I see in that thread. I already created a video about it, so, I might as well create a blog post too :)

P.S. You can try out the scripts in the DataWeave Playground or in Visual Studio Code.

The question

In summary, Harshank Bansal notices that if you try to run the following script, you will receive an error message stating that ABC cannot be coerced to Number (which is expected).


%dw 2.0
output application/json
"ABC" as Number


Cannot coerce String (ABC) to Number

4| "ABC" as Number
  at main::main (line: 4, column: 1)

Maybe your first instinct to try to fix this code - if you’re familiar with the try function - would be to do a script similar to the one Harshank mentions in the question:

%dw 2.0
import * from dw::Runtime
output application/json
try(() -> ("ABC" as Number)) orElse "Invalid number"

This correctly catches the error and outputs the “Invalid number” string instead. However, Harshank found an alternative for this specific use case which is using the default keyword. Like so:

%dw 2.0
output application/json
("ABC" as Number) default "Invalid number"

This previous script outputs the exact same “Invalid number” string as we did with the try function. However, this seems a bit weird since the default keyword is mainly used for null values and this is an error.

Now let’s talk about the answer/explanation that Mariano de Achaval wrote on why this behavior happens.

The answer

If you don’t know who Mariano is, he’s one of the architects that started developing DataWeave years ago. That’s why his answer is a big deal :)

Mariano mentions that the default keyword can be used for two things:

  1. When the left-side value is null, the right-side value is returned.

  2. To handle some exceptions like the coercion one we saw at the beginning of the post.

Notice how I wrote some in bold there, that’s because Mariano and Harshank both mention it doesn’t work for all possible exceptions. It only works when:

  1. You call a function that doesn’t support null.

  2. You try to coerce a value to a data type (like the example we saw).

Now, it is worth mentioning that this behavior is available to use but it is not recommended (as Mariano wrote). It is better to use the try function instead.

My takeaway

Even though it’s not recommended to use, it doesn’t hurt to know it exists :)

Sometimes you need to do a quick script where you need to ensure the type coercion doesn’t return an exception (like Harshank’s example) and the code isn’t really too complex to add the try function.

What is your takeaway from this??


1,599 views2 comments


Since both default and try have the same limitations, does anyone know how or if graceful error handling can be implemented? See the text below for details on why this is needed. We are currently writing a sizable dw lib and have implemented a few hundred unit tests. We have not yet successfully found any method to unit test unhappy flows, in fact it seems that certain errors, mostly weave exceptions and type mismatches simply crash the dw execution engine. In regular mule flows, this is sort of fine, if the payload is wrong, why should dw continue processing that payload? Well, the error could happen in an optional part of the message, and the rest might still be valid and should be…

Alex Martinez
Alex Martinez
Feb 04, 2023
Replying to

hey nicky! I'm sorry, I didn't get the notification about your comment.

this is very interesting and definitely worth discussing it more with the DW engineers! There are two options to continue this conversation with them:

1, Raise your concern in GitHub here:

2. Join the Slack workspace where you can have more dynamic discussions with the team. You can find the Slack link here:

Please feel free to contact me through LinkedIn or email in case I can help you with anything else!

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