Back-end: emit let instead of var. (#6102)

Fixes #856

Author: cristianoc

CHANGELOG.md

@@ -31,6 +31,7 @@
 - Remove empty export blocks in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6744
 - Fix indent for returned/thrown/wrapped in parentheses objects in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6746
 - Fix indent in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6747
+- In generated code, use `let` instead of `var`. https://github.com/rescript-lang/rescript-compiler/pull/6102
 
 # 11.1.0
 

jscomp/build_tests/custom_namespace/src/demo.bs.js

@@ -1,7 +1,7 @@
 'use strict';
 
 
-var v = 42;
+let v = 42;
 
 exports.v = v;
 /* No side effect */

jscomp/build_tests/devonly/src/demo.bs.js

@@ -182,7 +182,7 @@ let pp_paren_params (inner_cxt : cxt) (f : Ext_pp.t) (lexical : Ident.t list) :
       id) *)
 
 let pp_var_assign cxt f id =
-  P.string f L.var;
+  P.string f L.let_;
   P.space f;
   let acxt = Ext_pp_scope.ident cxt f id in
   P.space f;
@@ -199,7 +199,7 @@ let pp_var_assign_this cxt f id =
   cxt
 
 let pp_var_declare cxt f id =
-  P.string f L.var;
+  P.string f L.let_;
   P.space f;
   let acxt = Ext_pp_scope.ident cxt f id in
   semi f;

jscomp/build_tests/devonly/src/depdemo.bs.js

@@ -111,7 +111,7 @@ let requires require_lit cxt f (modules : (Ident.t * string * bool) list) =
   in
   P.at_least_two_lines f;
   Ext_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/build_tests/devonly/src2/hello.bs.js

@@ -29,7 +29,7 @@ let function_ = "function"
 
 let function_async ~async = if async then "async function" else "function"
 
-let var = "var" (* should be able to switch to [let] easily*)
+let let_ = "let"
 
 let return = "return"