The stored program language must be declared explicitly using LANGUAGE JAVASCRIPT in the CREATE FUNCTION or CREATE PROCEDURE statement used to create the stored program; otherwise, MySQL assumes the intended language is SQL.

The syntax of the routine body is checked at creation time; any errors cause the CREATE statement to be rejected, and the stored program not to be created.

The program body must be demarcated using the AS keyword plus dollar-quoted delimiters such as $$, $js$, $mysql$, and so on. You must use the same delimiter to mark both the beginning and end of the routine body. It is possible to use quotation marks to delimit the routine body, but dollar-quoted delimiters are preferred, since this avoids issues with quoting strings in function or procedure code. Following the first dollar-quoted delimiter, the mysql client prompt changes to $> for each new line within the routine body, until it reaches a closing dollar-quoted delimiter, after which the prompt reverts to the default (normally ->). This can be seen in the CREATE FUNCTION statement used to create the add_nos() function previously.

It is not necessary to specify a statement delimiter or terminator as it is for SQL stored routines. If you employ the optional ; character to separate JavaScript statements, this is interpeted correctly as being part of the JavaScript routine, and not as an SQL statement delimiter, as shown here:

mysql> CREATE FUNCTION js_pow(arg1 INT, arg2 INT) 
    -> RETURNS INT LANGUAGE JAVASCRIPT 
    -> AS 
    ->   $$
    $>     let x = Math.pow(arg1, arg2);
    $>     return x;
    $>   $$
    -> ;
Query OK, 0 rows affected (0.02 sec)

mysql> SELECT js_pow(2,3);
+-------------+
| js_pow(2,3) |
+-------------+
|           8 |
+-------------+
1 row in set (0.00 sec)

To minimise possible confusion, we do not use the ; separator for any JavaScript statements in the remaining examples in this section.

The text of the routine body is always treated as utfmb4 regardless of the character set actually used. What this means is that, whatever character set is used by the client for the routine body, the server converts it to utf8mb4 before storing it in the data dictionary. Since utf8mb4 subsumes all other supported character sets, this should not be an issue.

JavaScript program argument and routine names must use the utfmb3 character set, as with SQL stored programs. See Section 12.9, “Unicode Support”.

String arguments and return types are expected to be utf8mb4; this means that, if the default character set of the schema to which the JavaScript stored program belongs is some other character set, all of its arguments must be declared explicitly as utf8mb4.

Input argument names must adhere to the rules for JavaScript identifiers: They can contain Unicode letters, $, _, and digits (0-9), but may not start with a digit.

Using a word that is reserved in JavaScript (such as var or function) as the name of an argument raises an error. Since MySQL JavaScript stored programs always use strict mode, this also includes keywords such as package and let. See Reserved Words for a complete listing of these. In addition, the keywords mysql, console, and graal are also reserved by the MLE component, and cannot be used as variable or argument names in MySQL JavaScript stored programs.

You can modify a JavaScript stored program using ALTER FUNCTION and ALTER PROCEDURE as you would an SQL stored function or procedure. Changing the language using ALTER is not supported; in such cases, you must use DROP FUNCTION or DROP PROCEDURE as applicable, then re-create the stored program using the appopriate CREATE statement.