feat: support typeof on #private Fields (#2174)

CHANGELOG.md

@@ -2,6 +2,28 @@
 
 ## Unreleased
 
+* Add support for parsing `typeof` on #private fields from TypeScript 4.7 ([#2174](https://github.com/evanw/esbuild/pull/2174))
+
+    The upcoming version of TypeScript now lets you use `#private` fields in `typeof` type expressions:
+
+    https://devblogs.microsoft.com/typescript/announcing-typescript-4-7-beta/#typeof-on-private-fields
+
+    ```ts
+    class Container {
+      #data = "hello!";
+
+      get data(): typeof this.#data {
+        return this.#data;
+      }
+
+      set data(value: typeof this.#data) {
+        this.#data = value;
+      }
+    }
+    ```
+
+    With this release, esbuild can now parse these new type expressions as well. This feature was contributed by [@magic-akari](https://github.com/magic-akari).
+
 * Change TypeScript class field behavior when targeting ES2022
 
     TypeScript 4.3 introduced a breaking change where class field behavior changes from assign semantics to define semantics when the `target` setting in `tsconfig.json` is set to `ESNext`. Specifically, the default value for TypeScript's `useDefineForClassFields` setting when unspecified is `true` if and only if `target` is `ESNext`. TypeScript 4.6 introduced another change where this behavior now happens for both `ESNext` and `ES2022`. Presumably this will be the case for `ES2023` and up as well. With this release, esbuild's behavior has also been changed to match. Now configuring esbuild with `--target=es2022` will also cause TypeScript files to use the new class field behavior.

internal/js_parser/ts_parser.go

@@ -365,17 +365,21 @@ func (p *parser) skipTypeScriptTypeWithOpts(level js_ast.L, opts skipTypeOpts) {
 				continue
 			} else {
 				// "typeof x"
+				if !p.lexer.IsIdentifierOrKeyword() {
+					p.lexer.Expected(js_lexer.TIdentifier)
+				}
+				p.lexer.Next()
+
 				// "typeof x.y"
-				for {
-					if !p.lexer.IsIdentifierOrKeyword() {
-						p.lexer.Expected(js_lexer.TIdentifier)
-					}
+				// "typeof x.#y"
+				for p.lexer.Token == js_lexer.TDot {
 					p.lexer.Next()
-					if p.lexer.Token != js_lexer.TDot {
-						break
+					if !p.lexer.IsIdentifierOrKeyword() && p.lexer.Token != js_lexer.TPrivateIdentifier {
+						p.lexer.Expected(js_lexer.TIdentifier)
 					}
 					p.lexer.Next()
 				}
+
 				p.skipTypeScriptTypeArguments(false /* isInsideJSXElement */)
 			}
 

internal/js_parser/ts_parser_test.go

@@ -365,6 +365,9 @@ func TestTSTypes(t *testing.T) {
 	expectParseErrorTSX(t, "<in T extends any>() => {}", jsxErrorArrow)
 	expectParseErrorTSX(t, "<out T extends any>() => {}", jsxErrorArrow)
 	expectParseErrorTSX(t, "<in out T extends any>() => {}", jsxErrorArrow)
+	expectPrintedTS(t, "class Container { get data(): typeof this.#data {} }", "class Container {\n  get data() {\n  }\n}\n")
+	expectPrintedTS(t, "const a: typeof this.#a = 1;", "const a = 1;\n")
+	expectParseErrorTS(t, "const a: typeof #a = 1;", "<stdin>: ERROR: Expected identifier but found \"#a\"\n")
 }
 
 func TestTSAsCast(t *testing.T) {