Implement Statement.reader property for better-sqlite3 compatibility

Returns true if the statement returns data (has columns), false otherwise.
This allows checking whether a prepared statement is a reader (SELECT, or
INSERT/UPDATE/DELETE with RETURNING) before execution.

Author: penberg

compat.js

@@ -255,7 +255,7 @@ class Statement {
   }
 
   get reader() {
-    throw new Error("not implemented");
+    return this.stmt.columns().length > 0;
   }
 
   /**

docs/api.md

@@ -179,6 +179,10 @@ Toggle query duration timing.
 
 Returns the columns in the result set returned by this prepared statement.
 
+### reader ⇒ boolean
+
+Returns `true` if the statement returns data (i.e., it is a `SELECT` statement or an `INSERT`/`UPDATE`/`DELETE` with a `RETURNING` clause), `false` otherwise.
+
 ### bind([...bindParameters]) ⇒ this
 
 This function is currently not supported.

integration-tests/tests/async.test.js

@@ -410,6 +410,48 @@ test.serial("Concurrent writes over same connection", async (t) => {
   t.is(rows.length, 1000);
 });
 
+test.serial("Statement.reader [SELECT is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("SELECT * FROM users WHERE id = ?");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [INSERT is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("INSERT INTO users (name, email) VALUES (?, ?)");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [UPDATE is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("UPDATE users SET name = ? WHERE id = ?");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [DELETE is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("DELETE FROM users WHERE id = ?");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [INSERT RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("INSERT INTO users (name, email) VALUES (?, ?) RETURNING *");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [UPDATE RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("UPDATE users SET name = ? WHERE id = ? RETURNING *");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [DELETE RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = await db.prepare("DELETE FROM users WHERE id = ? RETURNING *");
+  t.is(stmt.reader, true);
+});
+
 const connect = async (path_opt, options = {}) => {
   const path = path_opt ?? "hello.db";
   const provider = process.env.PROVIDER;

integration-tests/tests/sync.test.js

@@ -457,6 +457,48 @@ test.serial("Timeout option", async (t) => {
   fs.unlinkSync(path);
 });
 
+test.serial("Statement.reader [SELECT is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("SELECT * FROM users WHERE id = ?");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [INSERT is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("INSERT INTO users (name, email) VALUES (?, ?)");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [UPDATE is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("UPDATE users SET name = ? WHERE id = ?");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [DELETE is false]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("DELETE FROM users WHERE id = ?");
+  t.is(stmt.reader, false);
+});
+
+test.serial("Statement.reader [INSERT RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("INSERT INTO users (name, email) VALUES (?, ?) RETURNING *");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [UPDATE RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("UPDATE users SET name = ? WHERE id = ? RETURNING *");
+  t.is(stmt.reader, true);
+});
+
+test.serial("Statement.reader [DELETE RETURNING is true]", async (t) => {
+  const db = t.context.db;
+  const stmt = db.prepare("DELETE FROM users WHERE id = ? RETURNING *");
+  t.is(stmt.reader, true);
+});
+
 const connect = async (path_opt, options = {}) => {
   const path = path_opt ?? "hello.db";
   const provider = process.env.PROVIDER;

promise.js

@@ -300,7 +300,7 @@ class Statement {
   }
 
   get reader() {
-    throw new Error("not implemented");
+    return this.stmt.columns().length > 0;
   }
 
   /**