JavaScript Syntax Checker

Parse JavaScript locally without running it. Compare classic-script and ES-module grammar, pinpoint line/column errors, inspect AST and token evidence, import local files, and export a privacy-safe diagnostic report.

Wave170 · parser-backed syntax evidence · no pasted code execution

JavaScript Syntax & Parse Diagnostics Studio

Parse a snippet or local file as a classic script or ES module, compare both grammar modes, pinpoint the first parser error with line/column context, and inspect AST/token evidence without executing the pasted program.

Bundled Acorn · local parse-only

1. Source

Import local file

Ready.

3. Result at a glance

Syntax—

Selected mode—

Error line:column—

AST nodes—

Tokens—

Selected parse—

Script vs module evidence

Grammar	Status	Evidence	Time

First-error code frame

4. AST summary

Most common node types

Node type	Count

Top-level statements

#	Type	Lines	Name / source

5. Supplemental lexical preflight

This lightweight pass can surface an unmatched bracket, string or block comment around end-of-input failures. The Acorn parser result remains authoritative for this Studio.

#	Line:col	Signal

6. Token preview

First 120 parser tokens only. This is an inspection aid, not a source formatter.

#	Type	Value	Start	End

7. Audit handoff & repeat use

Recent local summaries

Summary history stores only timestamp, valid/error status, selected grammar, line count, character count and error line. Your source is stored only when you explicitly choose “Save source session locally.” Settings-only links never include pasted code.

Truth boundary. This is a parse-only JavaScript syntax checker, not ESLint, TypeScript type checking, a formatter, runtime execution, dependency resolution, security review or proof that a program works. The bundled parser is a fixed Acorn 8.15.0 snapshot, so syntax added after that snapshot or proposal/JSX/TypeScript syntax can differ from current runtimes. Auto mode compares classic script and ES-module grammar; the browser-native cross-check is classic-script only and constructs but never calls a function.

Practical guide and verification

Syntax is only the first layer

A syntax checker can catch malformed tokens, brackets and grammar, but syntactically valid JavaScript can still fail at runtime because variables, APIs, modules or data are missing.

Environment matters

Browser scripts, Node.js modules and build-tool code do not always accept the same globals or module syntax. Check the code using the parser and language mode that matches the environment where it will actually run.

Common mistake

Do not interpret “syntax valid” as “safe” or “correct.” Logic errors, unsafe DOM use, injection risks, asynchronous failures and type coercion can remain in code that parses perfectly.

What this checker proves

A parser error means the selected JavaScript grammar cannot build a syntax tree for the source. A successful parse proves only grammar acceptance for the chosen parser snapshot and mode; undefined variables, bad API calls, type mistakes, race conditions, logic bugs and unsafe behavior can remain.

Script and module are different grammars

ES modules are strict by definition and permit import/export declarations. Classic scripts use script grammar. Auto mode runs both parsers and shows both outcomes so a module-only construct is not mislabeled as a generic syntax failure.

Fix the first parser error first

One missing quote or delimiter can move the parser far from the true mistake and create noisy follow-on symptoms. Start with the first line/column and code frame, repair it, then parse again.