diff --git a/test/test_jslib.py b/test/test_jslib.py
index 9f7cce742f2b7..c357fd0e6f791 100644
--- a/test/test_jslib.py
+++ b/test/test_jslib.py
@@ -39,8 +39,8 @@ def test_jslib_no_override(self):
     ''')
 
     self.cflags += ['--js-library', 'duplicated_func_1.js', '--js-library', 'duplicated_func_2.js']
-    err = self.expect_fail([EMCC, 'duplicated_func.c'] + self.get_cflags())
-    self.assertContained('duplicated_func_2.js: Symbol re-definition in JavaScript library: duplicatedFunc. Do not use noOverride if this is intended', err)
+    self.assert_fail([EMCC, 'duplicated_func.c'] + self.get_cflags(),
+                     'duplicated_func_2.js: Symbol re-definition in JavaScript library: duplicatedFunc. Do not use noOverride if this is intended')
 
   def test_jslib_missing_sig(self):
     create_file('some_func.c', '''
@@ -61,8 +61,8 @@ def test_jslib_missing_sig(self):
     ''')
 
     self.cflags += ['--js-library', 'some_func.js']
-    err = self.expect_fail([EMCC, 'some_func.c'] + self.get_cflags())
-    self.assertContained('some_func.js: __sig is missing for function: someFunc. Do not use checkSig if this is intended', err)
+    self.assert_fail([EMCC, 'some_func.c'] + self.get_cflags(),
+                     'some_func.js: __sig is missing for function: someFunc. Do not use checkSig if this is intended')
 
   def test_jslib_extra_args(self):
     # Verify that extra arguments in addition to those listed in `__sig` are still present
@@ -193,16 +193,14 @@ def test_jslib_preprocessor_errors(self):
 // This is a library file
 #endif // line 2
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'])
-    self.assertContained('lib.js:2: #endif without matching #if', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'], 'lib.js:2: #endif without matching #if')
 
     create_file('lib.js', '''\
 // This is a library file
 
 #else // line 3
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'])
-    self.assertContained('lib.js:3: #else without matching #if', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'], 'lib.js:3: #else without matching #if')
 
   def test_jslib_internal_deps(self):
     create_file('lib.js', r'''
@@ -252,8 +250,8 @@ def test_jslib_sig_redefinition(self):
   jslibfunc: (x) => {},
 });
 ''')
-    err = self.expect_fail([EMCC, 'src.c', '--js-library', 'lib.js', '--js-library', 'lib2.js'])
-    self.assertContained('lib2.js: signature redefinition for: jslibfunc__sig. (old=ii vs new=pp)', err)
+    self.assert_fail([EMCC, 'src.c', '--js-library', 'lib.js', '--js-library', 'lib2.js'],
+                     'lib2.js: signature redefinition for: jslibfunc__sig. (old=ii vs new=pp)')
 
   def test_jslib_invalid_deps(self):
     create_file('lib.js', r'''
@@ -263,8 +261,8 @@ def test_jslib_invalid_deps(self):
 });
 ''')
 
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'])
-    self.assertContained('lib.js: JS library directive jslibfunc__deps=hello is of type \'string\', but it should be an array', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'],
+                     'lib.js: JS library directive jslibfunc__deps=hello is of type \'string\', but it should be an array')
 
     create_file('lib2.js', r'''
 addToLibrary({
@@ -273,8 +271,8 @@ def test_jslib_invalid_deps(self):
 });
 ''')
 
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib2.js'])
-    self.assertContained("lib2.js: __deps entries must be of type 'string' or 'function' not 'number': jslibfunc__deps", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib2.js'],
+                     "lib2.js: __deps entries must be of type 'string' or 'function' not 'number': jslibfunc__deps")
 
   def test_jslib_invalid_decorator(self):
     create_file('lib.js', r'''
@@ -283,8 +281,8 @@ def test_jslib_invalid_decorator(self):
   jslibfunc: (x) => {},
 });
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'])
-    self.assertContained("lib.js: Decorator (jslibfunc__async} has wrong type. Expected 'boolean' not 'string'", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'lib.js'],
+                     "lib.js: Decorator (jslibfunc__async} has wrong type. Expected 'boolean' not 'string'")
 
   @also_with_wasm64
   @also_without_bigint
@@ -312,8 +310,8 @@ def test_jslib_i53abi_errors(self):
   jslibfunc: (x) => { return 42 },
 });
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'])
-    self.assertContained("error: JS library error: '__i53abi' decorator requires '__sig' decorator: 'jslibfunc'", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'],
+                     "error: JS library error: '__i53abi' decorator requires '__sig' decorator: 'jslibfunc'")
 
     create_file('lib.js', r'''
 mergeInto(LibraryManager.library, {
@@ -322,8 +320,8 @@ def test_jslib_i53abi_errors(self):
   jslibfunc: (x) => { return 42 },
 });
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'])
-    self.assertContained("error: JS library error: '__i53abi' only makes sense when '__sig' includes 'j' (int64): 'jslibfunc'", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'],
+                     "error: JS library error: '__i53abi' only makes sense when '__sig' includes 'j' (int64): 'jslibfunc'")
 
   def test_jslib_invalid_proxy_mode(self):
     create_file('lib.js', r'''
@@ -332,8 +330,8 @@ def test_jslib_invalid_proxy_mode(self):
   jslibfunc: (x) => 42,
 });
 ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'])
-    self.assertContained("error: JS library error: invalid proxying mode 'jslibfunc__proxy: foo' specified", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '-sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=jslibfunc', '--js-library', 'lib.js'],
+                     "error: JS library error: invalid proxying mode 'jslibfunc__proxy: foo' specified")
 
   def test_jslib_legacy(self):
     create_file('lib.js', r'''
@@ -357,7 +355,7 @@ def test_jslib_custom_settings(self):
     self.cflags += ['--js-library', test_file('jslib/test_jslib_custom_settings.js'), '-jsDCUSTOM_JS_OPTION=1']
     self.do_run_in_out_file_test('jslib/test_jslib_custom_settings.c')
 
-    self.assertContained('cannot change built-in settings values with a -jsD directive', self.expect_fail([EMCC, '-jsDWASM=0']))
+    self.assert_fail([EMCC, '-jsDWASM=0'], 'cannot change built-in settings values with a -jsD directive')
 
   def test_jslib_native_deps(self):
     # Verify that memset (which lives in compiled code), can be specified as a JS library
@@ -418,8 +416,7 @@ def test_jslib_bad_config(self):
         foo__sig: 'ii',
       });
       ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library=lib.js'])
-    self.assertContained("lib.js: Missing library element 'foo' for library config 'foo__sig'", err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library=lib.js'], "lib.js: Missing library element 'foo' for library config 'foo__sig'")
 
   def test_jslib_ifdef(self):
     create_file('lib.js', '''
@@ -463,8 +460,7 @@ def test_jslib_search_path(self):
     self.do_runf('main.c', '42\n', cflags=['-L.', '-lfoo.js'])
 
     # If the library path is not included with `-L` we expect the command to fail
-    err = self.expect_fail([EMCC, 'main.c', '-lfoo.js'])
-    self.assertContained('emcc: error: unable to find library -lfoo.js', err)
+    self.assert_fail([EMCC, 'main.c', '-lfoo.js'], 'emcc: error: unable to find library -lfoo.js')
 
   # Tests using the #warning directive in JS library files
   def test_jslib_warnings(self):
@@ -502,8 +498,7 @@ def test_jslib_include(self):
     self.run_process([EMCC, test_file('hello_world.c'), '--js-library', 'foo.js'])
 
     delete_file('inc.js')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', 'foo.js'])
-    self.assertContained('foo.js:5: file not found: inc.js', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library', 'foo.js'], 'foo.js:5: file not found: inc.js')
 
   @also_with_wasm64
   @also_without_bigint
@@ -609,8 +604,7 @@ def test_jslib_export_alias(self, args):
 
   def test_postjs_errors(self):
     create_file('post.js', '#preprocess\n#error This is an error')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--post-js', 'post.js'])
-    self.assertContained('post.js:2: #error This is an error', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--post-js', 'post.js'], 'post.js:2: #error This is an error')
 
   def test_jslib_has_library(self):
     create_file('libfoo.js', '''
@@ -655,8 +649,7 @@ def test_jslib_new_objects_non_empty(self):
         }
       });
       ''')
-    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library=lib.js', '-sEXPORTED_FUNCTIONS=obj,_main'])
-    self.assertContained('cannot stringify Map with data', err)
+    self.assert_fail([EMCC, test_file('hello_world.c'), '--js-library=lib.js', '-sEXPORTED_FUNCTIONS=obj,_main'], 'cannot stringify Map with data')
 
   def test_jslib_system_lib_name(self):
     create_file('libcore.js', r'''