feat: emit let instead of var

Changes.md

@@ -10,6 +10,8 @@ Unreleased
   ([#1036](https://github.com/melange-re/melange/pull/1036))
 - stdlib: fix runtime primitive for `Float.{min,max}` and related functions
   ([#1050](https://github.com/melange-re/melange/pull/1050))
+- core: emit `let` instead of `var` in compiled JS
+  ([#1019](https://github.com/melange-re/melange/pull/1019))
 
 3.0.0 2024-01-28
 ---------------

jscomp/core/js_dump.ml

@@ -224,7 +224,7 @@ let pp_paren_params (cxt : cxt) (lexical : Ident.t list) : unit =
        id) *)
 
 let pp_var_assign cxt id =
-  string cxt L.var;
+  string cxt L.let_;
   space cxt;
   let acxt = ident cxt id in
   space cxt;
@@ -241,7 +241,7 @@ let pp_var_assign_this cxt id =
   cxt
 
 let pp_var_declare cxt id =
-  string cxt L.var;
+  string cxt L.let_;
   space cxt;
   let acxt = ident cxt id in
   semi cxt;

jscomp/core/js_dump_import_export.ml

@@ -112,7 +112,7 @@ let requires require_lit cxt f (modules : (Ident.t * string * bool) list) =
   in
   P.at_least_two_lines f;
   List.rev_iter reversed_list (fun (s, file, default) ->
-      P.string f L.var;
+      P.string f L.let_;
       P.space f;
       P.string f s;
       P.space f;

jscomp/core/js_dump_lit.ml

@@ -23,7 +23,7 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. *)
 
 let function_ = "function"
-let var = "var" (* should be able to switch to [let] easily*)
+let let_ = "let"
 let return = "return"
 let require = "require"
 let import = "import"

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

@@ -1,7 +1,7 @@
 // Generated by Melange
 
 
-var $$default = 3;
+let $$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) {
-  var match = Stdlib__Array.to_list(Process.argv);
+  let match = Stdlib__Array.to_list(Process.argv);
   if (!match) {
     return false;
   }
-  var match$1 = match.tl;
+  let match$1 = match.tl;
   if (!match$1) {
     return false;
   }
-  var exec = Path.basename(match$1.hd);
+  let 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) {
-  var match = Stdlib__Array.to_list(Process.argv);
+  let match = Stdlib__Array.to_list(Process.argv);
   if (match && is_mocha(undefined)) {
     describe(name, (function () {
             return Stdlib__List.iter((function (param) {
-                          var partial_arg = param[1];
+                          let 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) {
-  var threshold = thresholdOpt !== undefined ? thresholdOpt : 0.0000001;
+  let 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 :
-        var b = spec._1;
-        var a = spec._0;
+        let b = spec._1;
+        let a = spec._0;
         if (!close_enough(undefined, a, b)) {
           Assert.deepEqual(a, b);
           return ;
         } else {
           return ;
         }
     case /* ApproxThreshold */6 :
-        var b$1 = spec._2;
-        var a$1 = spec._1;
+        let b$1 = spec._2;
+        let 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) {
-  var match = Stdlib__Array.to_list(Process.argv);
+  let match = Stdlib__Array.to_list(Process.argv);
   if (match) {
     if (is_mocha(undefined)) {
       describe(name, (function () {
               return Stdlib__List.iter((function (param) {
-                            var code = param[1];
+                            let 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) {
-                    var name = param[0];
-                    var _fn = Curry._1(param[1], undefined);
+                    let name = param[0];
+                    let _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) {
 
 }
 
-var val_unit = Promise.resolve(undefined);
+let val_unit = Promise.resolve(undefined);
 
 function from_promise_suites(name, suites) {
-  var match = Stdlib__Array.to_list(Process.argv);
+  let match = Stdlib__Array.to_list(Process.argv);
   if (match) {
     if (is_mocha(undefined)) {
       describe(name, (function () {
               return Stdlib__List.iter((function (param) {
-                            var code = param[1];
+                            let code = param[1];
                             it(param[0], (function () {
                                     return code.then(function (x) {
                                                 handleCode(x);