Write better code

JavaScript Code Checker

Check your JavaScript code security before your next PR commit and get alerts of critical bugs using our free online JavaScript code checker — powered by Snyk Code.

Sign up for free to unlock the the full power of Snyk, no credit card required.


How to use the free code checker


Copy and paste your JavaScript code into the editor.


Select your language from the dropdown.


Click the Check code button.


Use the results to improve your JavaScript code.

Get code security right from your IDE

This free code checker can find critical vulnerabilities and security issues with a click. To take your application security to the next level, we recommend using Snyk Code for free right from your IDE.


  • File I/O corruptions

  • API contract violations

  • Null dereferences

  • Process/threading deadlock problems

  • Incorrect type checking

  • Expression logic mistakes

  • Regular expression denial of service

  • Invalid time/date formatting

  • Resource leaks


  • Missing input data sanitization

  • Insecure password handling

  • Protocol insecurities

  • Indefensive permissions

  • Man-in-the-Middle attacks

  • Weak cryptography algorithms

  • Information disclosure

  • Code injection

  • SQL injection

JavaScript code security powered by Snyk Code

This free web based JavaScript code checker is powered by Snyk Code. Sign up now to get access to all the features including vulnerability alerts, real time scan results, and actionable fix advice within your IDE.

Human-in-the-Loop JavaScript Code Checker

Snyk Code is an expert-curated, AI-powered JavaScript code checker that analyzes your code for security issues, providing actionable advice directly from your IDE to help you fix vulnerabilities quickly.


Scan and fix source code in minutes.


Fix vulns with dev friendly remediation.

Integrated in IDE

Find vulns early to save time & money.


Integrates into existing workflow.

More than syntax errors

Comprehensive semantic analysis.

AI powered by people

Modern ML directed by security experts.

In-workflow testing

Automatically scan every PR and repo.

CI/CD security gate

Integrate scans into the build process.

Frequently asked questions

How to check JavaScript?

There are multiple methods for checking JavaScript code. Some IDEs support JavaScript code checking, allowing developers to easily perform a JavaScript error check. Additionally, a JavaScript validator or linter, such as ESLint, can parse code and compare it against a set of rules. Both of these methods include JavaScript syntax checkers that scan for syntax, formatting, and good coding practices. However, neither method identifies security vulnerabilities or provides detailed information for fixing errors. Development teams looking to check for security issues and fix errors quickly should rely on an AI-powered Javascript code checker.

What are the benefits of an AI-powered JavaScript code checker?

An AI-powered JavaScript code checker can surface syntax errors and code quality issues that impact the execution of a JavaScript application. These tools can use AI or machine learning algorithms that are trained to identify code that doesn’t follow best practices for security and quality. AI-powered JavaScript code checkers can often catch issues that aren’t identified during peer reviews or pair programming.

How to fix invalid JavaScript syntax?

Fixing invalid code syntax starts with using a code checker. These automated tools can provide additional information about syntax errors beyond the generic messages the JavaScript interpreter might give likeSyntaxError: unexpected stringorSyntaxError: unexpected token. It’s also a good idea to use a code checker or debugger to identify logical errors that may impact the JavaScript application during runtime. This can help developers catch and fix code issues before they reach production.

Common JavaScript syntax and logical errors

There are a variety of syntax and logical errors, so it’s important to know how to remediate the most common issues that a debugger or code checker may flag. Here are some best practices for avoiding common JavaScript syntax errors:

  • Declare and initialize variables at the top

  • Never declare number, string, or boolean objects

  • Beware of automatic type conversions in mathematical operations

  • Always end switch statements with a default value

  • Be sure to close every bracket and parenthesis

A JavaScript code checker is invaluable for finding and preventing bugs early on in the development process and

Logical errors aren’t recognized by the JavaScript interpreter, but they still prevent the application from performing as the developer originally intended. However, JavaScript error checkers look for logic mistakes. Here are some tips to avoid the logical errors that many developers regularly make when writing JavaScript code:

  • Remember that by default variables are undefined and objects are null

  • Avoid using the eval() function

  • Avoid using global variables and prioritize local variables

  • Do not use new Object()

  • Focus on breaking out of loops early

  • Avoid confusing the assignment (=) and equality operators (==, ===)

How to use a JavaScript Code Checker to improve code quality and security practices

A JavaScript code checker is invaluable for finding and preventing bugs early on in the development process and preventing developers from introducing JavaScript security vulnerabilities. Scanning tools are particularly useful for JavaScript because it’s an interpreted language, meaning there’s no compile process during development that flags syntax errors before execution. While some JavaScript engines do perform just-in-time compilation, this would surface errors much later in the development process.

Application security is another key aspect of checking JavaScript code. Using automated static analysis, development teams can find and remediate vulnerabilities early on in the development process rather than react to security incidents after deployment. This shift left of security can greatly reduce the risk exposure of applications and lower the cost of cybersecurity at many organizations.

Implementing an automated code checker with the development process, therefore, can dramatically improve the quality and security of JavaScript code without requiring a lot of extra effort by developers.

What is JavaScript code quality?

Measuring JavaScript code quality can vary depending on the best practices and standards a development team chooses to follow. Here are five common signs that any JavaScript code is high-quality.


It’s a best practice to write code that’s highly reusable. If your JavaScript code is easy to understand for other If your JavaScript code is easy to understand for other developers, it’s much easier to maintain in the long run. Understandable code uses consistent syntax, meaningful naming conventions, and consistent comments to ensure yourself and others know how the code works and why it’s needed. This is important when code that was written months or years before needs to be changed.

Functional code works as developers originally intended. That means the source code is well-tested and free from logical errors that could negatively affect the user experience later on. It’s also important to ensure the code is reliable to ensure the application has high availability, data integrity, and fault tolerance as well.

By writing concise code that’s understandable and functional, testing becomes much easier. You should focus on short, purposeful code blocks that can be tested using automated testing. In many cases, a test-driven approach with test cases written before the code can dramatically improve code quality as well.

High-quality JavaScript code should also be highly secure so that malicious actors cannot exploit the application and cause unwanted behavior. Scanning tools can detect vulnerabilities within code, configuration files, and more so that developers can minimize the security risks during development.

Maintaining JavaScript code is crucial to avoid issues later on. As a codebase grows and becomes more complicated, low quality code can introduce technical debt. If there’s technical debt, developers spend increasing amounts of time fixing bugs and security issues rather than building new functionality, which negatively impacts software innovation going forward.

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Snyk is a developer security platform. Integrating directly into development tools, workflows, and automation pipelines, Snyk makes it easy for teams to find, prioritize, and fix security vulnerabilities in code, dependencies, containers, and infrastructure as code. Supported by industry-leading application and security intelligence, Snyk puts security expertise in any developer’s toolkit.

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