diff --git a/lib/mjs/mjs_exec.c b/lib/mjs/mjs_exec.c
index 265e7d5c3..0213a871e 100644
--- a/lib/mjs/mjs_exec.c
+++ b/lib/mjs/mjs_exec.c
@@ -452,6 +452,12 @@ static int getprop_builtin_string(
     } else if(strcmp(name, "slice") == 0) {
         *res = mjs_mk_foreign_func(mjs, (mjs_func_ptr_t)mjs_string_slice);
         return 1;
+    } else if(strcmp(name, "toUpperCase") == 0) {
+        *res = mjs_mk_foreign_func(mjs, (mjs_func_ptr_t)mjs_string_to_upper_case);
+        return 1;
+    } else if(strcmp(name, "toLowerCase") == 0) {
+        *res = mjs_mk_foreign_func(mjs, (mjs_func_ptr_t)mjs_string_to_lower_case);
+        return 1;
     } else if(isnum) {
         /*
      * string subscript: return a new one-byte string if the index
diff --git a/lib/mjs/mjs_string.c b/lib/mjs/mjs_string.c
index f74bf1074..f771ff4ea 100644
--- a/lib/mjs/mjs_string.c
+++ b/lib/mjs/mjs_string.c
@@ -286,6 +286,41 @@ MJS_PRIVATE mjs_val_t s_concat(struct mjs* mjs, mjs_val_t a, mjs_val_t b) {
     return res;
 }
 
+MJS_PRIVATE void mjs_string_to_case(struct mjs* mjs, bool upper) {
+    mjs_val_t ret = MJS_UNDEFINED;
+    size_t size;
+    const char* s = NULL;
+
+    /* get string from `this` */
+    if(!mjs_check_arg(mjs, -1 /*this*/, "this", MJS_TYPE_STRING, NULL)) {
+        goto clean;
+    }
+    s = mjs_get_string(mjs, &mjs->vals.this_obj, &size);
+
+    if(size == 0) {
+        ret = mjs_mk_string(mjs, "", 0, 1);
+        goto clean;
+    }
+
+    char* tmp = malloc(size);
+    for(size_t i = 0; i < size; i++) {
+        tmp[i] = upper ? toupper(s[i]) : tolower(s[i]);
+    }
+    ret = mjs_mk_string(mjs, tmp, size, 1);
+    free(tmp);
+
+clean:
+    mjs_return(mjs, ret);
+}
+
+MJS_PRIVATE void mjs_string_to_lower_case(struct mjs* mjs) {
+    mjs_string_to_case(mjs, false);
+}
+
+MJS_PRIVATE void mjs_string_to_upper_case(struct mjs* mjs) {
+    mjs_string_to_case(mjs, true);
+}
+
 MJS_PRIVATE void mjs_string_slice(struct mjs* mjs) {
     int nargs = mjs_nargs(mjs);
     mjs_val_t ret = mjs_mk_number(mjs, 0);
diff --git a/lib/mjs/mjs_string.h b/lib/mjs/mjs_string.h
index ba6869b62..f0801f1a4 100644
--- a/lib/mjs/mjs_string.h
+++ b/lib/mjs/mjs_string.h
@@ -33,6 +33,8 @@ MJS_PRIVATE void embed_string(
 
 MJS_PRIVATE void mjs_mkstr(struct mjs* mjs);
 
+MJS_PRIVATE void mjs_string_to_lower_case(struct mjs* mjs);
+MJS_PRIVATE void mjs_string_to_upper_case(struct mjs* mjs);
 MJS_PRIVATE void mjs_string_slice(struct mjs* mjs);
 MJS_PRIVATE void mjs_string_index_of(struct mjs* mjs);
 MJS_PRIVATE void mjs_string_char_code_at(struct mjs* mjs);