From 0783585e3836d638ad6c8a76a7c80917d09a7653 Mon Sep 17 00:00:00 2001
From: Evan Wallace <evan.exe@gmail.com>
Date: Sat, 9 May 2020 09:05:41 -0700
Subject: [PATCH] always dead-code-eliminate optional chains (#28)

---
 internal/parser/parser.go      | 16 ++++++++++++----
 internal/parser/parser_test.go | 12 ++++++------
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/internal/parser/parser.go b/internal/parser/parser.go
index 0f7633cde2d..0e19c9e85cf 100644
--- a/internal/parser/parser.go
+++ b/internal/parser/parser.go
@@ -7224,6 +7224,7 @@ func (p *parser) lowerOptionalChain(expr ast.Expr, in exprIn, out exprOut, thisA
 	valueWhenUndefined := ast.Expr{expr.Loc, &ast.EUndefined{}}
 	endsWithPropertyAccess := false
 	startsWithCall := false
+	originalExpr := expr
 	chain := []ast.Expr{}
 	loc := expr.Loc
 
@@ -7275,6 +7276,13 @@ flatten:
 		return valueWhenUndefined, exprOut{}
 	}
 
+	// Don't lower this if we don't need to. This check must be done here instead
+	// of earlier so we can do the dead code elimination above when the target is
+	// null or undefined.
+	if p.target >= ES2020 {
+		return originalExpr, exprOut{}
+	}
+
 	// Step 2: Figure out if we need to capture the value for "this" for the
 	// initial ECall. This will be passed to ".call(this, ...args)" later.
 	var thisArg ast.Expr
@@ -7779,7 +7787,7 @@ func (p *parser) visitExprInOut(expr ast.Expr, in exprIn) (ast.Expr, exprOut) {
 		// Lower optional chaining if we're the top of the chain
 		containsOptionalChain := e.IsOptionalChain || out.childContainsOptionalChain
 		isEndOfChain := e.IsParenthesized || !in.hasChainParent
-		if p.target < ES2020 && containsOptionalChain && isEndOfChain {
+		if containsOptionalChain && isEndOfChain {
 			return p.lowerOptionalChain(expr, in, out, nil)
 		}
 
@@ -7824,7 +7832,7 @@ func (p *parser) visitExprInOut(expr ast.Expr, in exprIn) (ast.Expr, exprOut) {
 
 			// Lower optional chaining if present since we're guaranteed to be the
 			// end of the chain
-			if p.target < ES2020 && out.childContainsOptionalChain {
+			if out.childContainsOptionalChain {
 				return p.lowerOptionalChain(expr, in, out, nil)
 			}
 
@@ -7879,7 +7887,7 @@ func (p *parser) visitExprInOut(expr ast.Expr, in exprIn) (ast.Expr, exprOut) {
 		// Lower optional chaining if we're the top of the chain
 		containsOptionalChain := e.IsOptionalChain || out.childContainsOptionalChain
 		isEndOfChain := e.IsParenthesized || !in.hasChainParent
-		if p.target < ES2020 && containsOptionalChain && isEndOfChain {
+		if containsOptionalChain && isEndOfChain {
 			return p.lowerOptionalChain(expr, in, out, nil)
 		}
 
@@ -8012,7 +8020,7 @@ func (p *parser) visitExprInOut(expr ast.Expr, in exprIn) (ast.Expr, exprOut) {
 		// Lower optional chaining if we're the top of the chain
 		containsOptionalChain := e.IsOptionalChain || out.childContainsOptionalChain
 		isEndOfChain := e.IsParenthesized || !in.hasChainParent
-		if p.target < ES2020 && containsOptionalChain && isEndOfChain {
+		if containsOptionalChain && isEndOfChain {
 			result, out := p.lowerOptionalChain(expr, in, out, thisArgFunc)
 			if thisArgWrapFunc != nil {
 				result = thisArgWrapFunc(result)
diff --git a/internal/parser/parser_test.go b/internal/parser/parser_test.go
index bc7869c11b8..2d206768307 100644
--- a/internal/parser/parser_test.go
+++ b/internal/parser/parser_test.go
@@ -1477,13 +1477,13 @@ func TestLowerOptionalChain(t *testing.T) {
 	expectPrintedTarget(t, ES2020, "x?.[y]", "x?.[y];\n")
 	expectPrintedTarget(t, ES2020, "x?.(y)", "x?.(y);\n")
 
-	expectPrintedTarget(t, ES2020, "null?.x", "null?.x;\n")
-	expectPrintedTarget(t, ES2020, "null?.[x]", "null?.[x];\n")
-	expectPrintedTarget(t, ES2020, "null?.(x)", "null?.(x);\n")
+	expectPrintedTarget(t, ES2020, "null?.x", "void 0;\n")
+	expectPrintedTarget(t, ES2020, "null?.[x]", "void 0;\n")
+	expectPrintedTarget(t, ES2020, "null?.(x)", "void 0;\n")
 
-	expectPrintedTarget(t, ES2020, "undefined?.x", "(void 0)?.x;\n")
-	expectPrintedTarget(t, ES2020, "undefined?.[x]", "(void 0)?.[x];\n")
-	expectPrintedTarget(t, ES2020, "undefined?.(x)", "(void 0)?.(x);\n")
+	expectPrintedTarget(t, ES2020, "undefined?.x", "void 0;\n")
+	expectPrintedTarget(t, ES2020, "undefined?.[x]", "void 0;\n")
+	expectPrintedTarget(t, ES2020, "undefined?.(x)", "void 0;\n")
 
 	// Check multiple levels of nesting
 	expectPrintedTarget(t, ES2019, "a?.b?.c?.d", `var _a, _b;