Support `if` statements

Fix for yt-dlp/yt_dlp#6131
Closes #31509

3 files changed, 54 insertions, 3 deletions

diff --git a/test/test_jsinterp.py b/test/test_jsinterp.py
index 5121c8cf8..c47def737 100644
--- a/test/test_jsinterp.py
+++ b/test/test_jsinterp.py
@@ -158,6 +158,38 @@ class TestJSInterpreter(unittest.TestCase):
         self.assertEqual(jsi.call_function('z'), 5)
         self.assertEqual(jsi.call_function('y'), 2)
 
+    def test_if(self):
+        jsi = JSInterpreter('''
+        function x() {
+            let a = 9;
+            if (0==0) {a++}
+            return a
+        }''')
+        self.assertEqual(jsi.call_function('x'), 10)
+
+        jsi = JSInterpreter('''
+        function x() {
+            if (0==0) {return 10}
+        }''')
+        self.assertEqual(jsi.call_function('x'), 10)
+
+        jsi = JSInterpreter('''
+        function x() {
+            if (0!=0) {return 1}
+            else {return 10}
+        }''')
+        self.assertEqual(jsi.call_function('x'), 10)
+
+        """  # Unsupported
+        jsi = JSInterpreter('''
+        function x() {
+            if (0!=0) {return 1}
+            else if (1==0) {return 2}
+            else {return 10}
+        }''')
+        self.assertEqual(jsi.call_function('x'), 10)
+        """
+
     def test_for_loop(self):
         # function x() { a=0; for (i=0; i-10; i++) {a++} a }
         jsi = JSInterpreter('''
diff --git a/test/test_youtube_signature.py b/test/test_youtube_signature.py
index 4e678cae0..ac37ffa45 100644
--- a/test/test_youtube_signature.py
+++ b/test/test_youtube_signature.py
@@ -135,6 +135,10 @@ _NSIG_TESTS = [
         'https://www.youtube.com/s/player/5a3b6271/player_ias.vflset/en_US/base.js',
         'B2j7f_UPT4rfje85Lu_e', 'm5DmNymaGQ5RdQ',
     ),
+    (
+        'https://www.youtube.com/s/player/dac945fd/player_ias.vflset/en_US/base.js',
+        'o8BkRxXhuYsBCWi6RplPdP', '3Lx32v_hmzTm6A',
+    ),
 ]
 
 
diff --git a/youtube_dl/jsinterp.py b/youtube_dl/jsinterp.py
index 530a705b4..9a3b8d7f2 100644
--- a/youtube_dl/jsinterp.py
+++ b/youtube_dl/jsinterp.py
@@ -214,7 +214,7 @@ class JSInterpreter(object):
         def __init__(self, msg, *args, **kwargs):
             expr = kwargs.pop('expr', None)
             if expr is not None:
-                msg = '{0} in: {1!r:.100}'.format(msg.rstrip(), expr)
+                msg = '{0} in: {1!r}'.format(msg.rstrip(), expr[:100])
             super(JSInterpreter.Exception, self).__init__(msg, *args, **kwargs)
 
     @classmethod
@@ -268,7 +268,7 @@ class JSInterpreter(object):
                 elif in_quote == '/' and char in '[]':
                     in_regex_char_group = char == '['
             escaping = not escaping and in_quote and char == '\\'
-            after_op = not in_quote and (char in cls.OP_CHARS or char == '[' or (char.isspace() and after_op))
+            after_op = not in_quote and (char in cls.OP_CHARS or (char.isspace() and after_op))
 
             if char != delim[pos] or any(counters.values()) or in_quote:
                 pos = skipping = 0
@@ -301,7 +301,7 @@ class JSInterpreter(object):
         separated = list(cls._separate(expr, delim, 1))
 
         if len(separated) < 2:
-            raise cls.Exception('No terminating paren {delim} in {expr:.100}'.format(**locals()))
+            raise cls.Exception('No terminating paren {delim} in {expr}'.format(**locals()))
         return separated[0][1:].strip(), separated[1].strip()
 
     @staticmethod
@@ -428,10 +428,25 @@ class JSInterpreter(object):
 
         m = re.match(r'''(?x)
                 (?P<try>try)\s*\{|
+                (?P<if>if)\s*\(|
                 (?P<switch>switch)\s*\(|
                 (?P<for>for)\s*\(
                 ''', expr)
         md = m.groupdict() if m else {}
+        if md.get('if'):
+            cndn, expr = self._separate_at_paren(expr[m.end() - 1:])
+            if_expr, expr = self._separate_at_paren(expr.lstrip())
+            # TODO: "else if" is not handled
+            else_expr = None
+            m = re.match(r'else\s*{', expr)
+            if m:
+                else_expr, expr = self._separate_at_paren(expr[m.end() - 1:])
+            cndn = _js_ternary(self.interpret_expression(cndn, local_vars, allow_recursion))
+            ret, should_abort = self.interpret_statement(
+                if_expr if cndn else else_expr, local_vars, allow_recursion)
+            if should_abort:
+                return ret, True
+
         if md.get('try'):
             try_expr, expr = self._separate_at_paren(expr[m.end() - 1:])
             err = None