Further improve expression parsing in jsinterp, thx dirkf

From ytdl-org/youtube-dl@76ac699

force-test

src/jsinterp.py

@@ -736,16 +736,19 @@ def dict_item(key, val):
 			if op in ('+', '-'):
 				# simplify/adjust consecutive instances of these operators
 				undone = 0
-				while len(separated) > 1 and not separated[-1].strip():
+				separated = [s.strip() for s in separated]
+				while len(separated) > 1 and not separated[-1]:
 					undone += 1
 					separated.pop()
 				if op == '-' and undone % 2 != 0:
 					right_expr = op + right_expr
 				elif op == '+':
-					while len(separated) > 1 and separated[-1].strip() in self.OP_CHARS:
+					while len(separated) > 1 and set(separated[-1]) <= self.OP_CHARS:
+						right_expr = separated.pop() + right_expr
+					if separated[-1][-1:] in self.OP_CHARS:
 						right_expr = separated.pop() + right_expr
 				# hanging op at end of left => unary + (strip) or - (push right)
-				left_val = separated[-1]
+				left_val = separated[-1] if separated else ''
 				for dm_op in ('*', '%', '/', '**'):
 					bodmas = tuple(self._separate(left_val, dm_op, skip_delims=skip_delim))
 					if len(bodmas) > 1 and not bodmas[-1].strip():