diff --git a/runtime/wasm/ints.wat b/runtime/wasm/ints.wat
index f5c3fb8241..65f8a205b8 100644
--- a/runtime/wasm/ints.wat
+++ b/runtime/wasm/ints.wat
@@ -193,7 +193,7 @@
          (then (local.set $res (i32.sub (i32.const 0) (local.get $res)))))
       (local.get $res))
 
-   (#string $INT_ERRMSG "int.of_string")
+   (#string $INT_ERRMSG "int_of_string")
 
    (func (export "caml_int_of_string")
       (param $v (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/jslib.wat b/runtime/wasm/jslib.wat
index 03567900f4..625e13f31e 100644
--- a/runtime/wasm/jslib.wat
+++ b/runtime/wasm/jslib.wat
@@ -96,6 +96,10 @@
       (func $jsbytes_of_bytes (param (ref $bytes)) (result anyref)))
    (import "jsstring" "bytes_of_jsbytes"
       (func $bytes_of_jsbytes (param anyref) (result (ref $bytes))))
+   (import "jsstring" "jsstring_of_subbytes"
+      (func $jsstring_of_subbytes (param (ref $bytes) i32 i32) (result anyref)))
+   (import "jsstring" "jsstring_of_substring"
+      (func $jsstring_of_substring (param (ref eq) i32 i32) (result (ref eq))))
    (import "int32" "caml_copy_int32"
       (func $caml_copy_int32 (param i32) (result (ref eq))))
    (import "int32" "Int32_val"
@@ -476,6 +480,24 @@
       (return (struct.new $js (call $jsstring_of_bytes (local.get $s)))))
 ))
 
+(#if use-js-string
+(#then
+   (func (export "caml_jsstring_of_substring")
+      (param $s (ref eq)) (param $i i32) (param $l i32) (result (ref eq))
+      (return_call $jsstring_of_substring
+         (local.get $s) (local.get $i) (local.get $l)))
+)
+(#else
+   (func (export "caml_jsstring_of_substring")
+      (param $vs (ref eq)) (param $i i32) (param $l i32) (result (ref eq))
+      (local $s (ref $bytes))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vs)))
+      (return
+        (struct.new $js
+           (call $jsstring_of_subbytes
+              (local.get $s) (local.get $i) (local.get $l)))))
+))
+
 (#if use-js-string
 (#then
    (func (export "caml_jsbytes_of_string")
diff --git a/runtime/wasm/jsstring.wat b/runtime/wasm/jsstring.wat
index 6195e144b4..e81108b2e0 100644
--- a/runtime/wasm/jsstring.wat
+++ b/runtime/wasm/jsstring.wat
@@ -24,6 +24,8 @@
       (func $hash_string (param i32) (param anyref) (result i32)))
    (import "wasm:js-string" "length"
       (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "substring"
+      (func $string_sub (param externref i32 i32) (result (ref extern))))
    (import "wasm:js-string" "fromCharCodeArray"
       (func $fromCharCodeArray
          (param (ref null $wstring)) (param i32) (param i32)
@@ -172,7 +174,7 @@
                (br $loop))))
       (i32.const 1))
 
-   (func (export "jsstring_of_string")
+   (func $jsstring_of_string (export "jsstring_of_string")
       (param $s (ref eq)) (result (ref eq))
       (if (result (ref eq)) (call $string_is_ascii (local.get $s))
          (then
@@ -184,6 +186,18 @@
                      (struct.get $string 0
                         (ref.cast (ref $string) (local.get $s)))))))))
 
+   (func (export "jsstring_of_substring")
+      (param $s (ref eq)) (param $i i32) (param $l i32) (result (ref eq))
+      (return_call $jsstring_of_string
+         (struct.new $string
+            (any.convert_extern
+               (call $string_sub
+                  (extern.convert_any
+                     (struct.get $string 0
+                        (ref.cast (ref $string) (local.get $s))))
+                  (local.get $i)
+                  (i32.add (local.get $i) (local.get $l)))))))
+
    (func (export "string_of_jsstring")
       (param $s (ref eq)) (result (ref eq))
       (if (result (ref eq)) (call $string_is_ascii (local.get $s))