diff --git a/compiler/driver.c b/compiler/driver.c
index fd9826dc..1e165838 100644
--- a/compiler/driver.c
+++ b/compiler/driver.c
@@ -28,6 +28,10 @@ static int eq(const char * s1, const char * s2) {
     return strcmp(s1, s2) == 0;
 }
 
+static int startswith(const char * s1, const char * s2) {
+    return strncmp(s1, s2, strlen(s2)) == 0;
+}
+
 static void print_arglist(int exit_code) {
     FILE * f = exit_code ? stderr : stdout;
     fprintf(f, "Usage: snowball SOURCE_FILE... [OPTIONS]\n\n"
@@ -49,7 +53,8 @@ static void print_arglist(int exit_code) {
                "  -py, -python\n"
 #endif
 #ifndef DISABLE_JS
-               "  -js\n"
+               "  -js[=TYPE]                       generate Javascript (TYPE values:\n"
+               "                                   esm global, default: global)\n"
 #endif
 #ifndef DISABLE_RUST
                "  -rust\n"
@@ -115,6 +120,7 @@ static int read_options(struct options * o, int argc, char * argv[]) {
     o->output_file = 0;
     o->syntax_tree = false;
     o->comments = false;
+    o->js_esm = false;
     o->externals_prefix = NULL;
     o->variables_prefix = 0;
     o->runtime_path = 0;
@@ -162,6 +168,18 @@ static int read_options(struct options * o, int argc, char * argv[]) {
 #ifndef DISABLE_JS
             if (eq(s, "-js")) {
                 o->make_lang = LANG_JAVASCRIPT;
+                o->js_esm = false;
+                continue;
+            }
+            if (startswith(s, "-js=")) {
+                o->make_lang = LANG_JAVASCRIPT;
+                if (eq(s + 4, "global")) {
+                    o->js_esm = false;
+                } else if (eq(s + 4, "esm")) {
+                    o->js_esm = true;
+                } else {
+                    fprintf(stderr, "Unknown Javascript type '%s'\n", s + 4);
+                }
                 continue;
             }
 #endif
@@ -543,7 +561,11 @@ extern int main(int argc, char * argv[]) {
 #ifndef DISABLE_JS
                 if (o->make_lang == LANG_JAVASCRIPT) {
                     byte * s = add_sz_to_s(0, output_base);
-                    s = add_literal_to_s(s, ".js");
+                    if (o->js_esm) {
+                        s = add_literal_to_s(s, ".mjs");
+                    } else {
+                        s = add_literal_to_s(s, ".js");
+                    }
                     o->output_src = get_output(s);
                     lose_s(s);
                     generate_program_js(g);
diff --git a/compiler/generator_js.c b/compiler/generator_js.c
index bb4b20d3..fbbd156a 100644
--- a/compiler/generator_js.c
+++ b/compiler/generator_js.c
@@ -1221,11 +1221,21 @@ static void generate(struct generator * g, struct node * p) {
 }
 
 static void generate_class_begin(struct generator * g) {
-
-    w(g, "/**@constructor*/~N"
-         "var ~n = function() {~+~N"
-         "~Mconst ~P = require('./base-stemmer.js');~N"
-         "~Mvar base = new ~P();~N");
+    if (g->options->js_esm) {
+        w(g, "// deno-lint-ignore-file~N"
+             "import ~P from './base-stemmer.mjs'~N"
+             "~N"
+             "/** @typedef {{ stemWord(word: string): string }} Stemmer */~N"
+             "~N"
+             "/** @type {{ new(): Stemmer }} */~N"
+             "const ~n = function() {~+~N"
+             "~Mvar base = new ~P();~N");
+    } else {
+        w(g, "/**@constructor*/~N"
+             "var ~n = function() {~+~N"
+             "~Mconst ~P = require('./base-stemmer.js');~N"
+             "~Mvar base = new ~P();~N");
+    }
 }
 
 static void generate_class_end(struct generator * g) {
@@ -1237,8 +1247,13 @@ static void generate_class_end(struct generator * g) {
     w(g, "~Mreturn base.getCurrent();~N");
     w(g, "~-~M};~N");
     w(g, "~-};~N");
-    w(g, "~N");
-    w(g, "if (typeof module === 'object' && module.exports) module.exports = ~n;~N");
+    if (g->options->js_esm) {
+        w(g, "~N"
+             "export default ~n~N");
+    } else {
+        w(g, "~N"
+             "if (typeof module === 'object' && module.exports) module.exports = ~n;~N");
+    }
 }
 
 static void generate_among_table(struct generator * g, struct among * x) {
diff --git a/compiler/header.h b/compiler/header.h
index ff6f8078..6fd1b58c 100644
--- a/compiler/header.h
+++ b/compiler/header.h
@@ -385,6 +385,7 @@ struct options {
     FILE * output_h;
     byte syntax_tree;
     byte comments;
+    byte js_esm;
     enc encoding;
     enum { LANG_JAVA, LANG_C, LANG_CPLUSPLUS, LANG_CSHARP, LANG_PASCAL, LANG_PYTHON, LANG_JAVASCRIPT, LANG_RUST, LANG_GO, LANG_ADA } make_lang;
     const char * externals_prefix;
diff --git a/javascript/base-stemmer.js b/javascript/base-stemmer.js
index 63308cd7..d8003bc5 100644
--- a/javascript/base-stemmer.js
+++ b/javascript/base-stemmer.js
@@ -1,5 +1,18 @@
+// @ts-check
+
 /**@constructor*/
 const BaseStemmer = function() {
+    /** @protected */
+    this.current = '';
+    this.cursor = 0;
+    this.limit = 0;
+    this.limit_backward = 0;
+    this.bra = 0;
+    this.ket = 0;
+
+    /**
+     * @param {string} value
+     */
     this.setCurrent = function(value) {
         this.current = value;
         this.cursor = 0;
@@ -9,11 +22,18 @@ const BaseStemmer = function() {
         this.ket = this.limit;
     };
 
+    /**
+     * @return {string}
+     */
     this.getCurrent = function() {
         return this.current;
     };
 
+    /**
+     * @param {BaseStemmer} other
+     */
     this.copy_from = function(other) {
+        /** @protected */
         this.current          = other.current;
         this.cursor           = other.cursor;
         this.limit            = other.limit;
@@ -22,7 +42,14 @@ const BaseStemmer = function() {
         this.ket              = other.ket;
     };
 
+    /**
+     * @param {number[]} s
+     * @param {number} min
+     * @param {number} max
+     * @return {boolean}
+     */
     this.in_grouping = function(s, min, max) {
+        /** @protected */
         if (this.cursor >= this.limit) return false;
         var ch = this.current.charCodeAt(this.cursor);
         if (ch > max || ch < min) return false;
@@ -32,7 +59,14 @@ const BaseStemmer = function() {
         return true;
     };
 
+    /**
+     * @param {number[]} s
+     * @param {number} min
+     * @param {number} max
+     * @return {boolean}
+     */
     this.in_grouping_b = function(s, min, max) {
+        /** @protected */
         if (this.cursor <= this.limit_backward) return false;
         var ch = this.current.charCodeAt(this.cursor - 1);
         if (ch > max || ch < min) return false;
@@ -42,7 +76,14 @@ const BaseStemmer = function() {
         return true;
     };
 
+    /**
+     * @param {number[]} s
+     * @param {number} min
+     * @param {number} max
+     * @return {boolean}
+     */
     this.out_grouping = function(s, min, max) {
+        /** @protected */
         if (this.cursor >= this.limit) return false;
         var ch = this.current.charCodeAt(this.cursor);
         if (ch > max || ch < min) {
@@ -57,7 +98,14 @@ const BaseStemmer = function() {
         return false;
     };
 
+    /**
+     * @param {number[]} s
+     * @param {number} min
+     * @param {number} max
+     * @return {boolean}
+     */
     this.out_grouping_b = function(s, min, max) {
+        /** @protected */
         if (this.cursor <= this.limit_backward) return false;
         var ch = this.current.charCodeAt(this.cursor - 1);
         if (ch > max || ch < min) {
@@ -72,8 +120,13 @@ const BaseStemmer = function() {
         return false;
     };
 
+    /**
+     * @param {string} s
+     * @return {boolean}
+     */
     this.eq_s = function(s)
     {
+        /** @protected */
         if (this.limit - this.cursor < s.length) return false;
         if (this.current.slice(this.cursor, this.cursor + s.length) != s)
         {
@@ -83,8 +136,13 @@ const BaseStemmer = function() {
         return true;
     };
 
+    /**
+     * @param {string} s
+     * @return {boolean}
+     */
     this.eq_s_b = function(s)
     {
+        /** @protected */
         if (this.cursor - this.limit_backward < s.length) return false;
         if (this.current.slice(this.cursor - s.length, this.cursor) != s)
         {
@@ -94,8 +152,13 @@ const BaseStemmer = function() {
         return true;
     };
 
-    /** @return {number} */ this.find_among = function(v)
+    /**
+     * @param {Among[]} v
+     * @return {number}
+     */
+    this.find_among = function(v)
     {
+        /** @protected */
         var i = 0;
         var j = v.length;
 
@@ -165,8 +228,13 @@ const BaseStemmer = function() {
     };
 
     // find_among_b is for backwards processing. Same comments apply
+    /**
+     * @param {Among[]} v
+     * @return {number}
+     */
     this.find_among_b = function(v)
     {
+        /** @protected */
         var i = 0;
         var j = v.length
 
@@ -232,8 +300,15 @@ const BaseStemmer = function() {
     /* to replace chars between c_bra and c_ket in this.current by the
      * chars in s.
      */
+    /**
+     * @param {number} c_bra
+     * @param {number} c_ket
+     * @param {string} s
+     * @return {number}
+     */
     this.replace_s = function(c_bra, c_ket, s)
     {
+        /** @protected */
         var adjustment = s.length - (c_ket - c_bra);
         this.current = this.current.slice(0, c_bra) + s + this.current.slice(c_ket);
         this.limit += adjustment;
@@ -242,8 +317,12 @@ const BaseStemmer = function() {
         return adjustment;
     };
 
+    /**
+     * @return {boolean}
+     */
     this.slice_check = function()
     {
+        /** @protected */
         if (this.bra < 0 ||
             this.bra > this.ket ||
             this.ket > this.limit ||
@@ -254,8 +333,13 @@ const BaseStemmer = function() {
         return true;
     };
 
+    /**
+     * @param {number} c_bra
+     * @return {boolean}
+     */
     this.slice_from = function(s)
     {
+        /** @protected */
         var result = false;
         if (this.slice_check())
         {
@@ -265,20 +349,34 @@ const BaseStemmer = function() {
         return result;
     };
 
+    /**
+     * @return {boolean}
+     */
     this.slice_del = function()
     {
+        /** @protected */
         return this.slice_from("");
     };
 
+    /**
+     * @param {number} c_bra
+     * @param {number} c_ket
+     * @param {string} s
+     */
     this.insert = function(c_bra, c_ket, s)
     {
+        /** @protected */
         var adjustment = this.replace_s(c_bra, c_ket, s);
         if (c_bra <= this.bra) this.bra += adjustment;
         if (c_bra <= this.ket) this.ket += adjustment;
     };
 
+    /**
+     * @return {string}
+     */
     this.slice_to = function()
     {
+        /** @protected */
         var result = '';
         if (this.slice_check())
         {
@@ -287,8 +385,12 @@ const BaseStemmer = function() {
         return result;
     };
 
+    /**
+     * @return {string}
+     */
     this.assign_to = function()
     {
+        /** @protected */
         return this.current.slice(0, this.limit);
     };
 };