test,doc: clarify buf.indexOf(num) input range

Hopefully clarify the behaviour of `buffer.indexOf()` and
`buffer.includes()` for numbers in that they will be
truncated to uint8s.

Add tests for that behaviour.

Fixes: #7591
PR-URL: #7611
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/buffer.md

@@ -890,7 +890,8 @@ Operates similar to [`Array#indexOf()`][] in that it returns either the
 starting index position of `value` in Buffer or `-1` if the Buffer does not
 contain `value`. The `value` can be a String, Buffer or Number. Strings are by
 default interpreted as UTF8. Buffers will use the entire Buffer (to compare a
-partial Buffer use [`buf.slice()`][]).  Numbers can range from 0 to 255.
+partial Buffer use [`buf.slice()`][]). Numbers will be interpreted as unsigned 8-bit
+integer values between `0` and `255`.
 
 ```js
 const buf = new Buffer('this is a buffer');

test/parallel/test-buffer-indexof.js

@@ -302,3 +302,19 @@ assert.throws(function() {
 assert.throws(function() {
   b.indexOf([]);
 });
+
+// test truncation of Number arguments to uint8
+{
+  const buf = Buffer.from('this is a test');
+  assert.strictEqual(buf.indexOf(0x6973), 3);
+  assert.strictEqual(buf.indexOf(0x697320), 4);
+  assert.strictEqual(buf.indexOf(0x69732069), 2);
+  assert.strictEqual(buf.indexOf(0x697374657374), 0);
+  assert.strictEqual(buf.indexOf(0x69737374), 0);
+  assert.strictEqual(buf.indexOf(0x69737465), 11);
+  assert.strictEqual(buf.indexOf(0x69737465), 11);
+  assert.strictEqual(buf.indexOf(-140), 0);
+  assert.strictEqual(buf.indexOf(-152), 1);
+  assert.strictEqual(buf.indexOf(0xff), -1);
+  assert.strictEqual(buf.indexOf(0xffff), -1);
+}