Transformers
Last updated
Last updated
Transformers are designed for data transformation and reuse of your multi-line JavaScript code. Data from queries or components might not meet your needs in business scenarios. Also, you may use the same code block several times within an app. In such cases, a transformer is what you need.
Compared with inline code in {{ }}
, transformer supports multi-line code blocks. And unlike JavaScript query, transformer is designed to do read-only operations, which means that you cannot trigger a query or update a temporary state inside a transformer.
Click + New > Transfromer in a query editor to create a transformer.
Then write your JS code in the transformer. You can click Preview to get the return value and access it by transformerName.value
in your app.
In the following example, transformer1
uses the data of star rating in rating1
to calculate a score.
{{ }}
is disallowed inside a transformer or JS query. {{ }}
is only used for the purpose of single-line JS expression, whereas a transformer or JS query is for multiple lines of JS code.
Use the moment().format()
method to transform timestamp formats. The following example converts the timestamp value of start_time
returned by query1
to YYYY-MM-DD
format.
Use the _.orderBy()
method provided by lodash to sort data. The following example returns query1.data
sorted by amount
column in ascending order.
The example code below shows how to join query results of getUsers
and getOrders
on user id.
Only read-only operations are allowed inside a transformer. It means that you cannot set values of components or temporary states, or trigger queries. For those operations, use JavaScript queries instead.
For example, you cannot call the method setText()
of a text component in a transformer.
Instead, calling the method setText()
in a JavaScript query reports no error.
In another example, transformersort1
aims at sorting the data of getUsers
by first_name
, but the sort()
method may change the original data, so an error occurs.
In this case, use the method _.orderBy()
provided by lodash instead.