Transformers

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.

Quickstart

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 cases

Transform timestamp

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.

return query1.data.map(it => {
     return {
         ...it,
         start_time: moment(it.start_time).format('YYYY-MM-DD')
     };
})

Sort query data

Use the _.orderBy() method provided by lodash to sort data. The following example returns query1.data sorted by amount column in ascending order.

return _.orderBy(query1.data, 'amount', 'asc')

Join two queries

The example code below shows how to join query results of getUsers and getOrders on user id.

const users = getUsers.data;
const userOrders = getOrders.data;
return users.map(user => ({
  ...user,
  orders : userOrders.find(order => order.customer_id === user.id),
}));

Read-only operations

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.