Writing Couchbase extension functions in the JavaScript Language.
Introduction
SQL++ includes a large number of operations and generic functions that cover every aspect of data manipulation. In addition to the built-in functions, Couchbase also allows you to create your own extensions to the language.
Using user-defined functions, you can:
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Create reuseable, domain-specific functions for use in your applications.
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Execute complex logic that may be difficult to do in SQL++.
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Migrate from RDBMS stored procedures.
If you want to learn how to create JavaScript function libraries using the administration console and/or the REST-API then take a look at the JavaScript UDF Guides.
Added Constructs
JavaScript functions in Couchbase support most of the language constructs available in ECMAScript, though there are a number of restrictions related to the Couchbase environment. There are also additions that have been made to the language for working specifically with Couchbase.
SQL++ Embedded Statements
Top level SQL++ keywords, such as SELECT, UPDATE, INSERT and DELETE, are available as inline keywords in functions. Operations that return values such as SELECT are accessible through a returned iterable handle. SQL++ Query results, via a SELECT, are streamed in batches to the iterable handle as the iteration progresses through the result set.
function selectAirline(country) {
var q = SELECT name as airline_name, callsign as airline_callsign
FROM `travel-sample`.`inventory`.`airline`
WHERE country = $country; (1)
var res = [];
for (const doc of q) {
var airline = {}
airline.name = doc.airline_name (2)
airline.callsign = doc.airline_callsign (2)
res.push(airline);
}
return res;
}| 1 | The SQL++ is written directly into the JavaScript code without having to be used as part of a function call. You can even provide parameters that can be used in the SQL++ statement. |
| 2 | A standard JavaScript iterator is used to access the values returned from the SQL++ statement. |
Libraries and Scopes
JavaScript functions can be stored inside a library. A library can contain one or more functions, and can also be assigned to a scope, which allows libraries to be partitioned for logical grouping.
As shown in Figure 1, a JavaScript function library can exist as:
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A global library accessible across the cluster.
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A library accessible within a scope.
| You can find an introduction to scopes in the Couchbase Tutorials. |
Furthermore, access restrictions can be applied to scopes, so that only certain groups of users will be able to access collections and libraries within that scope.
You do not call a JavaScript function directly — for example, getBusinessDays(startDate, endDate) as shown here.
Instead, you must define a SQL++ User-Defined Function to act as a reference caller to the JavaScript function.
In Couchbase terminology, you would set the query context to travel-sample.inventory in order to run the functions in my-library.
Unsupported Features
Browser Extensions
Because JavaScript UDF functions do not execute in the context of a browser, the extensions that browsers add to the core language, such as window methods, DOM events etc. are not available.
Global State
All variables must be local to the function; global state is not permitted.
var count = 0; // Not allowed - global variable.
function increment() {
count++;
}Along with global state, global arrow functions are not supported. Arrow functions local to individual JavaScript functions are supported.