feat: emit const instead of let for immutable assignment

jscomp/core/js_dump.ml

@@ -225,6 +225,15 @@ let pp_var_assign cxt id =
   space cxt;
   acxt
 
+let pp_const_assign cxt id =
+  string cxt "const";
+  space cxt;
+  let acxt = ident cxt id in
+  space cxt;
+  string cxt L.eq;
+  space cxt;
+  acxt
+
 let pp_var_assign_this cxt id =
   let cxt = pp_var_assign cxt id in
   string cxt L.this;
@@ -333,6 +342,11 @@ let block_has_all_int_fields =
       !r
     with Local r -> r
 
+let pp_assign ~(property : Lam_group.let_kind) cxt name =
+  match property with
+  | Variable -> pp_var_assign cxt name
+  | Strict | Alias | StrictOpt -> pp_const_assign cxt name
+
 (* TODO: refactoring
    Note that {!pp_function} could print both statement and expression when [No_name] is given
 *)
@@ -383,7 +397,7 @@ and pp_function ~return_unit ~is_method cxt ~fn_state (l : Ident.t list)
       (* length *)
       match fn_state with
       | Name_top i | Name_non_top i ->
-          let cxt = pp_var_assign cxt i in
+          let cxt = pp_const_assign cxt i in
           let cxt = optimize len ~p:(arity = NA && len <= 8) cxt v in
           semi cxt;
           cxt
@@ -444,7 +458,7 @@ and pp_function ~return_unit ~is_method cxt ~fn_state (l : Ident.t list)
               space cxt;
               param_body ())
       | Name_non_top x ->
-          ignore (pp_var_assign inner_cxt x : cxt);
+          ignore (pp_const_assign inner_cxt x : cxt);
           string cxt L.function_;
           space cxt;
           param_body ();
@@ -894,7 +908,13 @@ and variable_declaration top cxt (variable : J.variable_declaration) : cxt =
   match variable with
   | { ident = i; value = None; ident_info; _ } ->
       if ident_info.used_stats = Dead_pure then cxt else pp_var_declare cxt i
-  | { ident = name; value = Some e; ident_info = { used_stats; _ }; _ } -> (
+  | {
+   ident = name;
+   value = Some e;
+   ident_info = { used_stats; _ };
+   property;
+   _;
+  } -> (
       match used_stats with
       | Dead_pure -> cxt
       | Dead_non_pure ->
@@ -907,7 +927,7 @@ and variable_declaration top cxt (variable : J.variable_declaration) : cxt =
                 ~fn_state:(if top then Name_top name else Name_non_top name)
                 params b env
           | _ ->
-              let cxt = pp_var_assign cxt name in
+              let cxt = pp_assign ~property cxt name in
               let cxt = expression ~level:1 cxt e in
               semi cxt;
               cxt))

jscomp/test/dist-es6/jscomp/test/es6_tests/es6_export.mjs

@@ -1,7 +1,7 @@
 // Generated by Melange
 
 
-let $$default = 3;
+const $$default = 3;
 
 export {
   $$default as default,

jscomp/test/dist-es6/jscomp/test/mt.mjs

@@ -12,15 +12,15 @@ function assert_fail(msg) {
 }
 
 function is_mocha(param) {
-  let match = Stdlib__Array.to_list(Process.argv);
+  const match = Stdlib__Array.to_list(Process.argv);
   if (!match) {
     return false;
   }
-  let match$1 = match.tl;
+  const match$1 = match.tl;
   if (!match$1) {
     return false;
   }
-  let exec = Path.basename(match$1.hd);
+  const exec = Path.basename(match$1.hd);
   if (exec === "mocha") {
     return true;
   } else {
@@ -29,11 +29,11 @@ function is_mocha(param) {
 }
 
 function from_suites(name, suite) {
-  let match = Stdlib__Array.to_list(Process.argv);
+  const match = Stdlib__Array.to_list(Process.argv);
   if (match && is_mocha(undefined)) {
     describe(name, (function () {
             return Stdlib__List.iter((function (param) {
-                          let partial_arg = param[1];
+                          const partial_arg = param[1];
                           it(param[0], (function () {
                                   return Curry._1(partial_arg, undefined);
                                 }));
@@ -45,7 +45,7 @@ function from_suites(name, suite) {
 }
 
 function close_enough(thresholdOpt, a, b) {
-  let threshold = thresholdOpt !== undefined ? thresholdOpt : 0.0000001;
+  const threshold = thresholdOpt !== undefined ? thresholdOpt : 0.0000001;
   return Math.abs(a - b) < threshold;
 }
 
@@ -67,17 +67,17 @@ function handleCode(spec) {
         Assert.ok(spec._0);
         return ;
     case /* Approx */5 :
-        let b = spec._1;
-        let a = spec._0;
+        const b = spec._1;
+        const a = spec._0;
         if (!close_enough(undefined, a, b)) {
           Assert.deepEqual(a, b);
           return ;
         } else {
           return ;
         }
     case /* ApproxThreshold */6 :
-        let b$1 = spec._2;
-        let a$1 = spec._1;
+        const b$1 = spec._2;
+        const a$1 = spec._1;
         if (!close_enough(spec._0, a$1, b$1)) {
           Assert.deepEqual(a$1, b$1);
           return ;
@@ -96,12 +96,12 @@ function handleCode(spec) {
 }
 
 function from_pair_suites(name, suites) {
-  let match = Stdlib__Array.to_list(Process.argv);
+  const match = Stdlib__Array.to_list(Process.argv);
   if (match) {
     if (is_mocha(undefined)) {
       describe(name, (function () {
               return Stdlib__List.iter((function (param) {
-                            let code = param[1];
+                            const code = param[1];
                             it(param[0], (function () {
                                     return handleCode(Curry._1(code, undefined));
                                   }));
@@ -114,8 +114,8 @@ function from_pair_suites(name, suites) {
             "testing"
           ]);
       return Stdlib__List.iter((function (param) {
-                    let name = param[0];
-                    let _fn = Curry._1(param[1], undefined);
+                    const name = param[0];
+                    const _fn = Curry._1(param[1], undefined);
                     switch (_fn.TAG | 0) {
                       case /* Eq */0 :
                           console.log([
@@ -191,15 +191,15 @@ function from_pair_suites(name, suites) {
 
 }
 
-let val_unit = Promise.resolve(undefined);
+const val_unit = Promise.resolve(undefined);
 
 function from_promise_suites(name, suites) {
-  let match = Stdlib__Array.to_list(Process.argv);
+  const match = Stdlib__Array.to_list(Process.argv);
   if (match) {
     if (is_mocha(undefined)) {
       describe(name, (function () {
               return Stdlib__List.iter((function (param) {
-                            let code = param[1];
+                            const code = param[1];
                             it(param[0], (function () {
                                     return code.then(function (x) {
                                                 handleCode(x);