Fix some panics in String.prototype properties (#888)

* Fix some panics in String.prototype properties

* Address review comments

boa/src/builtins/string/mod.rs

@@ -189,7 +189,8 @@ impl String {
         let primitive_val = this.to_string(ctx)?;
         let pos = args
             .get(0)
-            .expect("failed to get argument for String method")
+            .cloned()
+            .unwrap_or_else(Value::undefined)
             .to_integer(ctx)? as i32;
 
         // Calling .len() on a string would give the wrong result, as they are bytes not the number of
@@ -235,7 +236,8 @@ impl String {
         let length = primitive_val.chars().count();
         let pos = args
             .get(0)
-            .expect("failed to get argument for String method")
+            .cloned()
+            .unwrap_or_else(Value::undefined)
             .to_integer(ctx)? as i32;
 
         if pos >= length as i32 || pos < 0 {
@@ -325,20 +327,21 @@ impl String {
         // Then we convert it into a Rust String by wrapping it in from_value
         let primitive_val = this.to_string(ctx)?;
 
+        // Calling .len() on a string would give the wrong result, as they are bytes not the number of unicode code points
+        // Note that this is an O(N) operation (because UTF-8 is complex) while getting the number of bytes is an O(1) operation.
+        let length = primitive_val.chars().count() as i32;
+
         let start = args
             .get(0)
-            .expect("failed to get argument for String method")
+            .cloned()
+            .unwrap_or_else(Value::undefined)
             .to_integer(ctx)? as i32;
-
         let end = args
             .get(1)
-            .expect("failed to get argument in slice")
+            .cloned()
+            .unwrap_or_else(|| Value::integer(length))
             .to_integer(ctx)? as i32;
 
-        // Calling .len() on a string would give the wrong result, as they are bytes not the number of unicode code points
-        // Note that this is an O(N) operation (because UTF-8 is complex) while getting the number of bytes is an O(1) operation.
-        let length = primitive_val.chars().count() as i32;
-
         let from = if start < 0 {
             max(length.wrapping_add(start), 0)
         } else {

boa/src/builtins/string/tests.rs

@@ -766,6 +766,34 @@ fn last_index_non_integer_position_argument() {
     assert_eq!(forward(&mut engine, "'abcx'.lastIndexOf('x', null)"), "3");
 }
 
+#[test]
+fn char_at() {
+    let mut engine = Context::new();
+    assert_eq!(forward(&mut engine, "'abc'.charAt(1)"), "\"b\"");
+    assert_eq!(forward(&mut engine, "'abc'.charAt(9)"), "\"\"");
+    assert_eq!(forward(&mut engine, "'abc'.charAt()"), "\"a\"");
+    assert_eq!(forward(&mut engine, "'abc'.charAt(null)"), "\"a\"");
+}
+
+#[test]
+fn char_code_at() {
+    let mut engine = Context::new();
+    assert_eq!(forward(&mut engine, "'abc'.charCodeAt(1)"), "98");
+    assert_eq!(forward(&mut engine, "'abc'.charCodeAt(9)"), "NaN");
+    assert_eq!(forward(&mut engine, "'abc'.charCodeAt()"), "97");
+    assert_eq!(forward(&mut engine, "'abc'.charCodeAt(null)"), "97");
+}
+
+#[test]
+fn slice() {
+    let mut engine = Context::new();
+    assert_eq!(forward(&mut engine, "'abc'.slice()"), "\"abc\"");
+    assert_eq!(forward(&mut engine, "'abc'.slice(1)"), "\"bc\"");
+    assert_eq!(forward(&mut engine, "'abc'.slice(-1)"), "\"c\"");
+    assert_eq!(forward(&mut engine, "'abc'.slice(0, 9)"), "\"abc\"");
+    assert_eq!(forward(&mut engine, "'abc'.slice(9, 10)"), "\"\"");
+}
+
 #[test]
 fn empty_iter() {
     let mut engine = Context::new();