diff --git a/src/lib/js/string.rs b/src/lib/js/string.rs
index 85586afc51..cf6ee0576b 100644
--- a/src/lib/js/string.rs
+++ b/src/lib/js/string.rs
@@ -51,3 +51,14 @@ pub fn _create(global: Value) -> Value {
 pub fn init(global: Value) {
     global.set_field_slice("String", _create(global.clone()));
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn check_string_constructor_is_function() {
+        let global = ValueData::new_obj(None);
+        let string_constructor = _create(global);
+        assert_eq!(string_constructor.is_function(), true);
+    }
+}
diff --git a/src/lib/js/value.rs b/src/lib/js/value.rs
index 0d8ed0d01f..250d533fcc 100644
--- a/src/lib/js/value.rs
+++ b/src/lib/js/value.rs
@@ -763,3 +763,39 @@ pub fn from_value<A: FromValue>(v: Value) -> Result<A, &'static str> {
 pub fn to_value<A: ToValue>(v: A) -> Value {
     v.to_value()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn check_is_object() {
+        let val = ValueData::new_obj(None);
+        assert_eq!(val.is_object(), true);
+    }
+
+    #[test]
+    fn check_string_to_value() {
+        let s = String::from("Hello");
+        let v = s.to_value();
+        assert_eq!(v.is_string(), true);
+        assert_eq!(v.is_null(), false);
+    }
+
+    #[test]
+    fn check_undefined() {
+        let u = ValueData::Undefined;
+        assert_eq!(u.get_type(), "undefined");
+        assert_eq!(u.to_string(), "undefined");
+    }
+
+    #[test]
+    fn check_get_set_field() {
+        let obj = ValueData::new_obj(None);
+        // Create string and convert it to a Value
+        let s = String::from("bar").to_value();
+        obj.set_field_slice("foo", s);
+        assert_eq!(obj.get_field_slice("foo").to_string(), "bar");
+    }
+
+}
diff --git a/src/lib/syntax/lexer.rs b/src/lib/syntax/lexer.rs
index f4a375bcda..a7e386bdfa 100644
--- a/src/lib/syntax/lexer.rs
+++ b/src/lib/syntax/lexer.rs
@@ -441,3 +441,72 @@ impl<'a> Lexer<'a> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::syntax::ast::keyword::Keyword;
+
+    #[test]
+    fn check_variable_definition_tokens() {
+        let s = &String::from("let a = 'hello';");
+        let mut lexer = Lexer::new(s);
+        lexer.lex().expect("finished");
+        assert_eq!(lexer.tokens[0].data, TokenData::Keyword(Keyword::Let));
+        assert_eq!(lexer.tokens[1].data, TokenData::Identifier("a".to_string()));
+        assert_eq!(
+            lexer.tokens[2].data,
+            TokenData::Punctuator(Punctuator::Assign)
+        );
+        assert_eq!(
+            lexer.tokens[3].data,
+            TokenData::StringLiteral("hello".to_string())
+        );
+    }
+
+    #[test]
+    fn check_positions() {
+        let s = &String::from("console.log(\"hello world\");");
+        // -------------------123456789
+        let mut lexer = Lexer::new(s);
+        lexer.lex().expect("finished");
+        // The first column is 1 (not zero indexed)
+        assert_eq!(lexer.tokens[0].pos.column_number, 1);
+        assert_eq!(lexer.tokens[0].pos.line_number, 1);
+        // Dot Token starts on line 7
+        assert_eq!(lexer.tokens[1].pos.column_number, 8);
+        assert_eq!(lexer.tokens[1].pos.line_number, 1);
+        // Log Token starts on line 7
+        assert_eq!(lexer.tokens[2].pos.column_number, 9);
+        assert_eq!(lexer.tokens[2].pos.line_number, 1);
+        // Open parenthesis token starts on line 12
+        assert_eq!(lexer.tokens[3].pos.column_number, 12);
+        assert_eq!(lexer.tokens[3].pos.line_number, 1);
+        // String token starts on line 13
+        assert_eq!(lexer.tokens[4].pos.column_number, 13);
+        assert_eq!(lexer.tokens[4].pos.line_number, 1);
+        // Close parenthesis token starts on line 26
+        assert_eq!(lexer.tokens[5].pos.column_number, 26);
+        assert_eq!(lexer.tokens[5].pos.line_number, 1);
+        // Semi Colon token starts on line 27
+        assert_eq!(lexer.tokens[6].pos.column_number, 27);
+        assert_eq!(lexer.tokens[6].pos.line_number, 1);
+    }
+
+    // Increment/Decrement
+    #[test]
+    fn check_decrement_advances_lexer_2_places() {
+        // Here we want an example of decrementing an integer
+        let s = &String::from("let a = b--;");
+        let mut lexer = Lexer::new(s);
+        lexer.lex().expect("finished");
+        assert_eq!(lexer.tokens[4].data, TokenData::Punctuator(Punctuator::Dec));
+        // Decrementing means adding 2 characters '--', the lexer should consume it as a single token
+        // and move the curser forward by 2, meaning the next token should be a semicolon
+        assert_eq!(
+            lexer.tokens[5].data,
+            TokenData::Punctuator(Punctuator::Semicolon)
+        );
+    }
+
+}
diff --git a/tests/lexer_test.rs b/tests/lexer_test.rs
deleted file mode 100644
index 316d35ea10..0000000000
--- a/tests/lexer_test.rs
+++ /dev/null
@@ -1,70 +0,0 @@
-extern crate boa;
-
-use boa::syntax::ast::keyword::Keyword;
-use boa::syntax::ast::punc::Punctuator;
-use boa::syntax::ast::token::TokenData;
-use boa::syntax::lexer::Lexer;
-
-#[test]
-/// Check basic variable definition tokens
-fn check_variable_definition_tokens() {
-    let s = &String::from("let a = 'hello';");
-    let mut lexer = Lexer::new(s);
-    lexer.lex().expect("finished");
-    assert_eq!(lexer.tokens[0].data, TokenData::Keyword(Keyword::Let));
-    assert_eq!(lexer.tokens[1].data, TokenData::Identifier("a".to_string()));
-    assert_eq!(
-        lexer.tokens[2].data,
-        TokenData::Punctuator(Punctuator::Assign)
-    );
-    assert_eq!(
-        lexer.tokens[3].data,
-        TokenData::StringLiteral("hello".to_string())
-    );
-}
-
-#[test]
-/// Check positions are correct
-fn check_positions() {
-    let s = &String::from("console.log(\"hello world\");");
-    // -------------------123456789
-    let mut lexer = Lexer::new(s);
-    lexer.lex().expect("finished");
-    // The first column is 1 (not zero indexed)
-    assert_eq!(lexer.tokens[0].pos.column_number, 1);
-    assert_eq!(lexer.tokens[0].pos.line_number, 1);
-    // Dot Token starts on line 7
-    assert_eq!(lexer.tokens[1].pos.column_number, 8);
-    assert_eq!(lexer.tokens[1].pos.line_number, 1);
-    // Log Token starts on line 7
-    assert_eq!(lexer.tokens[2].pos.column_number, 9);
-    assert_eq!(lexer.tokens[2].pos.line_number, 1);
-    // Open parenthesis token starts on line 12
-    assert_eq!(lexer.tokens[3].pos.column_number, 12);
-    assert_eq!(lexer.tokens[3].pos.line_number, 1);
-    // String token starts on line 13
-    assert_eq!(lexer.tokens[4].pos.column_number, 13);
-    assert_eq!(lexer.tokens[4].pos.line_number, 1);
-    // Close parenthesis token starts on line 26
-    assert_eq!(lexer.tokens[5].pos.column_number, 26);
-    assert_eq!(lexer.tokens[5].pos.line_number, 1);
-    // Semi Colon token starts on line 27
-    assert_eq!(lexer.tokens[6].pos.column_number, 27);
-    assert_eq!(lexer.tokens[6].pos.line_number, 1);
-}
-
-// Increment/Decrement
-#[test]
-fn check_decrement_advances_lexer_2_places() {
-    // Here we want an example of decrementing an integer
-    let s = &String::from("let a = b--;");
-    let mut lexer = Lexer::new(s);
-    lexer.lex().expect("finished");
-    assert_eq!(lexer.tokens[4].data, TokenData::Punctuator(Punctuator::Dec));
-    // Decrementing means adding 2 characters '--', the lexer should consume it as a single token
-    // and move the curser forward by 2, meaning the next token should be a semicolon
-    assert_eq!(
-        lexer.tokens[5].data,
-        TokenData::Punctuator(Punctuator::Semicolon)
-    );
-}
diff --git a/tests/string_test.rs b/tests/string_test.rs
deleted file mode 100644
index 5f05fa45e6..0000000000
--- a/tests/string_test.rs
+++ /dev/null
@@ -1,11 +0,0 @@
-extern crate boa;
-use boa::js::string::_create;
-use boa::js::object::PROTOTYPE;
-use boa::js::value::{ValueData};
-
-#[test]
-fn check_string_constructor_is_function() {
-    let global = ValueData::new_obj(None);
-    let string_constructor = _create(global);
-    assert_eq!(string_constructor.is_function(), true);
-}
\ No newline at end of file
diff --git a/tests/value_test.rs b/tests/value_test.rs
deleted file mode 100644
index 09cd0225cf..0000000000
--- a/tests/value_test.rs
+++ /dev/null
@@ -1,32 +0,0 @@
-extern crate boa;
-use boa::js::value::{ToValue, ValueData};
-
-#[test]
-fn check_is_object() {
-    let val = ValueData::new_obj(None);
-    assert_eq!(val.is_object(), true);
-}
-
-#[test]
-fn check_string_to_value() {
-    let s = String::from("Hello");
-    let v = s.to_value();
-    assert_eq!(v.is_string(), true);
-    assert_eq!(v.is_null(), false);
-}
-
-#[test]
-fn check_undefined() {
-    let u = ValueData::Undefined;
-    assert_eq!(u.get_type(), "undefined");
-    assert_eq!(u.to_string(), "undefined");
-}
-
-#[test]
-fn check_get_set_field() {
-    let obj = ValueData::new_obj(None);
-    // Create string and convert it to a Value
-    let s = String::from("bar").to_value();
-    obj.set_field_slice("foo", s);
-    assert_eq!(obj.get_field_slice("foo").to_string(), "bar");
-}