[JavaScript] Add supports of StringBuilder.Chars

Fix #3759

src/Fable.Cli/CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+#### JavaScript
+
+* [GH-3759](https://github.com/fable-compiler/Fable/issues/3759) Add `StringBuilder.Chars` (by @MangelMaxime)
+
 ## 4.12.2 - 2024-02-13
 
 ### Changed

src/fable-library-ts/CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+#### JavaScript
+
+* [GH-3759](https://github.com/fable-compiler/Fable/issues/3759) Add `StringBuilder.Chars` (by @MangelMaxime)
+
 ## 1.0.0 - 2024-02-13
 
 * Release stable version

src/fable-library-ts/System.Text.fs

@@ -64,6 +64,28 @@ type StringBuilder(value: string, capacity: int) =
         buf.Add(System.Environment.NewLine)
         x
 
+    member x.Clear() =
+        buf.Clear()
+        x
+
+    member x.Chars
+        with get (index: int) =
+            let bufferText = x.ToString()
+
+            if index < 0 || index >= bufferText.Length then
+                failwith "Index was outside the bounds of the array"
+            else
+                bufferText.[index]
+        and set (index: int) (value: char) =
+            let bufferText = x.ToString()
+
+            if index < 0 || index >= bufferText.Length then
+                failwith "Index was outside the bounds of the array"
+            else
+                let bufferText = bufferText.ToCharArray()
+                bufferText.[index] <- value
+                x.Clear().Append(bufferText) |> ignore
+
     member x.Replace(oldValue: char, newValue: char) =
         for i = buf.Count - 1 downto 0 do
             buf[i] <- buf[i].Replace(oldValue, newValue)
@@ -89,7 +111,3 @@ type StringBuilder(value: string, capacity: int) =
     member x.ToString(firstIndex: int, length: int) =
         let str = x.ToString()
         str.Substring(firstIndex, length)
-
-    member x.Clear() =
-        buf.Clear()
-        x

tests/Js/Main/StringTests.fs

@@ -103,6 +103,38 @@ let tests =
                               .Append(34)
             equal "aaabcd/true5.234" (builder.ToString().Replace(",", ".").ToLower())
 
+      testCase "StringBuilder.Chars works" <| fun () ->
+            let builder = Text.StringBuilder()
+                              .Append("abc")
+            equal 'b' (builder.Chars(1))
+
+      testCase "StringBuilder.Chars throws when index is out of bounds" <| fun () ->
+            throwsAnyError <| fun () ->
+                let builder = Text.StringBuilder()
+                                  .Append("abc")
+                builder.Chars(-1) |> ignore
+
+                builder.Chars(3) |> ignore
+
+      testCase "StringBuilder.Replace works" <| fun () ->
+            let builder = Text.StringBuilder()
+                              .Append("abc")
+                              .Append("abc")
+                              .Replace('a', 'x')
+                              .Replace("bc", "yz")
+            equal "xyzxyz" (builder.ToString())
+
+      testCase "StringBuilder index accessor works" <| fun () ->
+            let builder = Text.StringBuilder()
+                              .Append("abc")
+            equal 'b' (builder[1])
+
+      testCase "StringBuilder index setter works" <| fun () ->
+            let builder = Text.StringBuilder()
+                              .Append("abc")
+            builder[1] <- 'x'
+            equal "axc" (builder.ToString())
+
       testCase "kprintf works" <| fun () ->
             let f (s:string) = s + "XX"
             Printf.kprintf f "hello" |> equal "helloXX"