diff --git a/boa/src/builtins/string/mod.rs b/boa/src/builtins/string/mod.rs
index 566188ae61..dd9abc8a3c 100644
--- a/boa/src/builtins/string/mod.rs
+++ b/boa/src/builtins/string/mod.rs
@@ -543,39 +543,25 @@ impl String {
     /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.indexof
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/indexOf
     pub(crate) fn index_of(this: &Value, args: &[Value], ctx: &mut Interpreter) -> ResultValue {
-        // First we get it the actual string a private field stored on the object only the engine has access to.
-        // Then we convert it into a Rust String by wrapping it in from_value
-        let primitive_val = ctx.to_string(this)?;
-
-        // TODO: Should throw TypeError if search_string is regular expression
-        let search_string = ctx.to_string(
-            args.get(0)
-                .expect("failed to get argument for String method"),
-        )?;
-
-        let length = primitive_val.chars().count() as i32;
-
-        // If less than 2 args specified, position is 'undefined', defaults to 0
-        let position = if args.len() < 2 {
-            0
-        } else {
-            i32::from(args.get(1).expect("Could not get argument"))
-        };
+        let this = ctx.require_object_coercible(this)?;
+        let string = ctx.to_string(this)?;
 
-        let start = min(max(position, 0), length);
+        let search_string =
+            ctx.to_string(&args.get(0).cloned().unwrap_or_else(Value::undefined))?;
 
-        // Here cannot use the &str method "find", because this returns the byte
-        // index: we need to return the char index in the JS String
-        // Instead, iterate over the part we're checking until the slice we're
-        // checking "starts with" the search string
-        for index in start..length {
-            let this_string: StdString = primitive_val.chars().skip(index as usize).collect();
-            if this_string.starts_with(search_string.as_str()) {
-                // Explicitly return early with the index value
-                return Ok(Value::from(index));
+        let length = string.chars().count();
+        let start = args
+            .get(1)
+            .map(|position| ctx.to_integer(position))
+            .transpose()?
+            .map_or(0, |position| position.max(0.0).min(length as f64) as usize);
+
+        if start < length {
+            if let Some(position) = string.find(search_string.as_str()) {
+                return Ok(string[..position].chars().count().into());
             }
         }
-        // Didn't find a match, so return -1
+
         Ok(Value::from(-1))
     }
 
diff --git a/boa/src/builtins/string/tests.rs b/boa/src/builtins/string/tests.rs
index b73e353229..aecc203b39 100644
--- a/boa/src/builtins/string/tests.rs
+++ b/boa/src/builtins/string/tests.rs
@@ -415,3 +415,85 @@ fn trim_end() {
     assert_eq!(forward(&mut engine, "'Hello \n'.trimEnd()"), "\"Hello\"");
     assert_eq!(forward(&mut engine, "' Hello '.trimEnd()"), "\" Hello\"");
 }
+
+#[test]
+fn index_of_with_no_arguments() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    assert_eq!(forward(&mut engine, "''.indexOf()"), "-1");
+    assert_eq!(forward(&mut engine, "'undefined'.indexOf()"), "0");
+    assert_eq!(forward(&mut engine, "'a1undefined'.indexOf()"), "2");
+    assert_eq!(forward(&mut engine, "'a1undefined1a'.indexOf()"), "2");
+    assert_eq!(forward(&mut engine, "'µµµundefined'.indexOf()"), "3");
+    assert_eq!(forward(&mut engine, "'µµµundefinedµµµ'.indexOf()"), "3");
+}
+
+#[test]
+fn index_of_with_string_search_string_argument() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    assert_eq!(forward(&mut engine, "''.indexOf('hello')"), "-1");
+    assert_eq!(
+        forward(&mut engine, "'undefined'.indexOf('undefined')"),
+        "0"
+    );
+    assert_eq!(
+        forward(&mut engine, "'a1undefined'.indexOf('undefined')"),
+        "2"
+    );
+    assert_eq!(
+        forward(&mut engine, "'a1undefined1a'.indexOf('undefined')"),
+        "2"
+    );
+    assert_eq!(
+        forward(&mut engine, "'µµµundefined'.indexOf('undefined')"),
+        "3"
+    );
+    assert_eq!(
+        forward(&mut engine, "'µµµundefinedµµµ'.indexOf('undefined')"),
+        "3"
+    );
+}
+
+#[test]
+fn index_of_with_non_string_search_string_argument() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    assert_eq!(forward(&mut engine, "''.indexOf(1)"), "-1");
+    assert_eq!(forward(&mut engine, "'1'.indexOf(1)"), "0");
+    assert_eq!(forward(&mut engine, "'true'.indexOf(true)"), "0");
+    assert_eq!(forward(&mut engine, "'ab100ba'.indexOf(100)"), "2");
+    assert_eq!(forward(&mut engine, "'µµµfalse'.indexOf(true)"), "-1");
+    assert_eq!(forward(&mut engine, "'µµµ5µµµ'.indexOf(5)"), "3");
+}
+
+#[test]
+fn index_of_with_from_index_argument() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    assert_eq!(forward(&mut engine, "''.indexOf('x', 2)"), "-1");
+    assert_eq!(forward(&mut engine, "'x'.indexOf('x', 2)"), "-1");
+    assert_eq!(forward(&mut engine, "'abcx'.indexOf('x', 2)"), "3");
+    assert_eq!(forward(&mut engine, "'x'.indexOf('x', 2)"), "-1");
+    assert_eq!(forward(&mut engine, "'µµµxµµµ'.indexOf('x', 2)"), "3");
+
+    assert_eq!(
+        forward(&mut engine, "'µµµxµµµ'.indexOf('x', 10000000)"),
+        "-1"
+    );
+}
+
+#[test]
+fn generic_index_of() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    forward_val(
+        &mut engine,
+        "Number.prototype.indexOf = String.prototype.indexOf",
+    )
+    .unwrap();
+
+    assert_eq!(forward(&mut engine, "(10).indexOf(9)"), "-1");
+    assert_eq!(forward(&mut engine, "(10).indexOf(0)"), "1");
+    assert_eq!(forward(&mut engine, "(10).indexOf('0')"), "1");
+}