#2380: allow declare class field initializers

Author: evanw

CHANGELOG.md

@@ -60,6 +60,33 @@
 
     This fix was contributed by [@magic-akari](https://github.com/magic-akari).
 
+* Allow `declare` class fields to be initialized ([#2380](https://github.com/evanw/esbuild/issues/2380))
+
+    This release fixes an oversight in the TypeScript parser that disallowed initializers for `declare` class fields. TypeScript actually allows the following limited initializer expressions for `readonly` fields:
+
+    ```ts
+    declare const enum a { b = 0 }
+
+    class Foo {
+      // These are allowed by TypeScript
+      declare readonly a = 0
+      declare readonly b = -0
+      declare readonly c = 0n
+      declare readonly d = -0n
+      declare readonly e = 'x'
+      declare readonly f = `x`
+      declare readonly g = a.b
+      declare readonly h = a['b']
+
+      // These are not allowed by TypeScript
+      declare readonly x = (0)
+      declare readonly y = null
+      declare readonly z = -a.b
+    }
+    ```
+
+    So with this release, esbuild now allows initializers for `declare` class fields too. To future-proof this in case TypeScript allows more expressions as initializers in the future (such as `null`), esbuild will allow any expression as an initializer and will leave the specifics of TypeScript's special-casing here to the TypeScript type checker.
+
 * Fix a bug in esbuild's feature compatibility table generator ([#2365](https://github.com/evanw/esbuild/issues/2365))
 
     Passing specific JavaScript engines to esbuild's `--target` flag restricts esbuild to only using JavaScript features that are supported on those engines in the output files that esbuild generates. The data for this feature is automatically derived from this compatibility table with a script: https://kangax.github.io/compat-table/.

internal/js_parser/js_parser.go

@@ -1984,10 +1984,6 @@ func (p *parser) parseProperty(startLoc logger.Loc, kind js_ast.PropertyKind, op
 		}
 
 		if p.lexer.Token == js_lexer.TEquals {
-			if opts.tsDeclareRange.Len != 0 {
-				p.log.AddError(&p.tracker, p.lexer.Range(), "Class fields that use \"declare\" cannot be initialized")
-			}
-
 			p.lexer.Next()
 
 			// "this" and "super" property access is allowed in field initializers

internal/js_parser/ts_parser_test.go

@@ -510,7 +510,7 @@ func TestTSClass(t *testing.T) {
 	expectPrintedTS(t, "class Foo { override public foo: number }", "class Foo {\n}\n")
 	expectPrintedTS(t, "class Foo { public override foo: number }", "class Foo {\n}\n")
 	expectPrintedTS(t, "class Foo { declare override public foo: number }", "class Foo {\n}\n")
-	expectParseErrorTS(t, "class Foo { declare foo = 123 }", "<stdin>: ERROR: Class fields that use \"declare\" cannot be initialized\n")
+	expectPrintedTS(t, "class Foo { declare foo = 123 }", "class Foo {\n}\n")
 
 	expectPrintedTS(t, "class Foo { public static foo: number }", "class Foo {\n}\n")
 	expectPrintedTS(t, "class Foo { private static foo: number }", "class Foo {\n}\n")
@@ -524,8 +524,8 @@ func TestTSClass(t *testing.T) {
 	expectPrintedTS(t, "class Foo { public override static foo: number }", "class Foo {\n}\n")
 	expectPrintedTS(t, "class Foo { public static override foo: number }", "class Foo {\n}\n")
 	expectPrintedTS(t, "class Foo { declare override public static foo: number }", "class Foo {\n}\n")
-	expectParseErrorTS(t, "class Foo { declare static foo = 123 }", "<stdin>: ERROR: Class fields that use \"declare\" cannot be initialized\n")
-	expectParseErrorTS(t, "class Foo { static declare foo = 123 }", "<stdin>: ERROR: Class fields that use \"declare\" cannot be initialized\n")
+	expectPrintedTS(t, "class Foo { declare static foo = 123 }", "class Foo {\n}\n")
+	expectPrintedTS(t, "class Foo { static declare foo = 123 }", "class Foo {\n}\n")
 
 	expectParseErrorTS(t, "class Foo { declare #foo }", "<stdin>: ERROR: \"declare\" cannot be used with a private identifier\n")
 	expectParseErrorTS(t, "class Foo { declare [foo: string]: number }", "<stdin>: ERROR: \"declare\" cannot be used with an index signature\n")