evanw/esbuild · error · LexerPanic

Unexpected "," after rest pattern

Error message

Unexpected "," after rest pattern

What it means

In array destructuring binding, a rest element '...rest' must be the last element and cannot be followed by a comma. When the parser sees a TComma right after a spread in the array-binding loop (js_parser.go:6066), it errors.

Source

Thrown at internal/js_parser/js_parser.go:6068

				p.saveExprCommentsHere()
				binding := p.parseBinding(parseBindingOpts{})

				var defaultValueOrNil js_ast.Expr
				if !hasSpread && p.lexer.Token == js_lexer.TEquals {
					p.lexer.Next()
					defaultValueOrNil = p.parseExpr(js_ast.LComma)
				}

				items = append(items, js_ast.ArrayBinding{
					Binding:           binding,
					DefaultValueOrNil: defaultValueOrNil,
					Loc:               itemLoc,
				})

				// Commas after spread elements are not allowed
				if hasSpread && p.lexer.Token == js_lexer.TComma {
					p.log.AddError(&p.tracker, p.lexer.Range(), "Unexpected \",\" after rest pattern")
					panic(js_lexer.LexerPanic{})
				}
			}

			if p.lexer.Token != js_lexer.TComma {
				break
			}
			if p.lexer.HasNewlineBefore {
				isSingleLine = false
			}
			p.lexer.Next()
			if p.lexer.HasNewlineBefore {
				isSingleLine = false
			}
		}

		p.allowIn = oldAllowIn

		if p.lexer.HasNewlineBefore {

View on GitHub (pinned to f6058f8364)

Solutions

1. Remove the comma and any elements after '...rest'.
2. Make the rest element truly last in the array pattern.

Example fix

// before
const [a, ...rest,] = arr
// after
const [a, ...rest] = arr

import "github.com/evanw/esbuild/pkg/api"
res := api.Transform(src, api.TransformOptions{Loader: api.LoaderJS})
for _, m := range res.Errors {
  if strings.Contains(m.Text, "Unexpected \",\" after rest pattern") {
    // a trailing comma appeared after ...rest in an array binding
  }
}

// Flag '...rest,' (comma after rest) inside an array destructuring binding.
var reRestCommaArray = regexp.MustCompile(`\\[[^\\]]*\\.\\.\\.[^\\]]*,\\s*[\\]\\)]`)
func arrayRestHasTrailingComma(src string) bool { return reRestCommaArray.MatchString(src) }

When it happens

Trigger: const [a, ...rest,] = arr or ([a, ...b, c] = x) — a comma (and any further elements) after the rest in an array pattern.

Common situations: A trailing comma after spread; additional elements accidentally placed after the rest; formatter inserting trailing commas.

Related errors

• Invalid binding pattern
• Unexpected newline before "=>"
• For loop initializers cannot start with "async of"
• Unexpected ":"
• Unexpected "..."

AI-assisted analysis of evanw/esbuild@f6058f8364 (2026-08-09). Data as JSON: /api/errors/ca006dd9ca9f5f8d. Report an issue: GitHub.