From 167d7a9f0ffd1b4fe600193441bdb7358db2740b Mon Sep 17 00:00:00 2001
From: sepro <sepro@sepr0.com>
Date: Thu, 22 May 2025 00:27:07 +0200
Subject: [PATCH] [jsinterp] Fix increment/decrement evaluation (#13238)

Closes #13241
Authored by: seproDev, bashonly

Co-authored-by: bashonly <88596187+bashonly@users.noreply.github.com>
---
 test/test_jsinterp.py          |  8 ++++++
 test/test_youtube_signature.py |  4 +++
 yt_dlp/jsinterp.py             | 49 +++++++++++++++++-----------------
 3 files changed, 37 insertions(+), 24 deletions(-)

diff --git a/test/test_jsinterp.py b/test/test_jsinterp.py
index b14069ccc6..2e3cdc2a59 100644
--- a/test/test_jsinterp.py
+++ b/test/test_jsinterp.py
@@ -478,6 +478,14 @@ class TestJSInterpreter(unittest.TestCase):
         func = jsi.extract_function('c', {'e': 10}, {'f': 100, 'g': 1000})
         self.assertEqual(func([1]), 1111)
 
+    def test_increment_decrement(self):
+        self._test('function f() { var x = 1; return ++x; }', 2)
+        self._test('function f() { var x = 1; return x++; }', 1)
+        self._test('function f() { var x = 1; x--; return x }', 0)
+        self._test('function f() { var y; var x = 1; x++, --x, x--, x--, y="z", "abc", x++; return --x }', -1)
+        self._test('function f() { var a = "test--"; return a; }', 'test--')
+        self._test('function f() { var b = 1; var a = "b--"; return a; }', 'b--')
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/test/test_youtube_signature.py b/test/test_youtube_signature.py
index 0f0885366e..3f777aed7a 100644
--- a/test/test_youtube_signature.py
+++ b/test/test_youtube_signature.py
@@ -316,6 +316,10 @@ _NSIG_TESTS = [
         'https://www.youtube.com/s/player/8a8ac953/tv-player-es6.vflset/tv-player-es6.js',
         'MiBYeXx_vRREbiCCmh', 'RtZYMVvmkE0JE',
     ),
+    (
+        'https://www.youtube.com/s/player/59b252b9/player_ias.vflset/en_US/base.js',
+        'D3XWVpYgwhLLKNK4AGX', 'aZrQ1qWJ5yv5h',
+    ),
 ]
 
 
diff --git a/yt_dlp/jsinterp.py b/yt_dlp/jsinterp.py
index b59fb2c615..45aeffa229 100644
--- a/yt_dlp/jsinterp.py
+++ b/yt_dlp/jsinterp.py
@@ -590,6 +590,29 @@ class JSInterpreter:
                     return ret, True
             return ret, False
 
+        m = re.match(fr'''(?x)
+                (?P<out>{_NAME_RE})(?:\[(?P<index>{_NESTED_BRACKETS})\])?\s*
+                (?P<op>{"|".join(map(re.escape, set(_OPERATORS) - _COMP_OPERATORS))})?
+                =(?!=)(?P<expr>.*)$
+            ''', expr)
+        if m:  # We are assigning a value to a variable
+            left_val = local_vars.get(m.group('out'))
+
+            if not m.group('index'):
+                local_vars[m.group('out')] = self._operator(
+                    m.group('op'), left_val, m.group('expr'), expr, local_vars, allow_recursion)
+                return local_vars[m.group('out')], should_return
+            elif left_val in (None, JS_Undefined):
+                raise self.Exception(f'Cannot index undefined variable {m.group("out")}', expr)
+
+            idx = self.interpret_expression(m.group('index'), local_vars, allow_recursion)
+            if not isinstance(idx, (int, float)):
+                raise self.Exception(f'List index {idx} must be integer', expr)
+            idx = int(idx)
+            left_val[idx] = self._operator(
+                m.group('op'), self._index(left_val, idx), m.group('expr'), expr, local_vars, allow_recursion)
+            return left_val[idx], should_return
+
         for m in re.finditer(rf'''(?x)
                 (?P<pre_sign>\+\+|--)(?P<var1>{_NAME_RE})|
                 (?P<var2>{_NAME_RE})(?P<post_sign>\+\+|--)''', expr):
@@ -606,11 +629,7 @@ class JSInterpreter:
             return None, should_return
 
         m = re.match(fr'''(?x)
-            (?P<assign>
-                (?P<out>{_NAME_RE})(?:\[(?P<index>{_NESTED_BRACKETS})\])?\s*
-                (?P<op>{"|".join(map(re.escape, set(_OPERATORS) - _COMP_OPERATORS))})?
-                =(?!=)(?P<expr>.*)$
-            )|(?P<return>
+            (?P<return>
                 (?!if|return|true|false|null|undefined|NaN)(?P<name>{_NAME_RE})$
             )|(?P<attribute>
                 (?P<var>{_NAME_RE})(?:
@@ -622,25 +641,7 @@ class JSInterpreter:
             )|(?P<function>
                 (?P<fname>{_NAME_RE})\((?P<args>.*)\)$
             )''', expr)
-        if m and m.group('assign'):
-            left_val = local_vars.get(m.group('out'))
-
-            if not m.group('index'):
-                local_vars[m.group('out')] = self._operator(
-                    m.group('op'), left_val, m.group('expr'), expr, local_vars, allow_recursion)
-                return local_vars[m.group('out')], should_return
-            elif left_val in (None, JS_Undefined):
-                raise self.Exception(f'Cannot index undefined variable {m.group("out")}', expr)
-
-            idx = self.interpret_expression(m.group('index'), local_vars, allow_recursion)
-            if not isinstance(idx, (int, float)):
-                raise self.Exception(f'List index {idx} must be integer', expr)
-            idx = int(idx)
-            left_val[idx] = self._operator(
-                m.group('op'), self._index(left_val, idx), m.group('expr'), expr, local_vars, allow_recursion)
-            return left_val[idx], should_return
-
-        elif expr.isdigit():
+        if expr.isdigit():
             return int(expr), should_return
 
         elif expr == 'break':