Javascript check syntax error

SyntaxError

The SyntaxError object represents an error when trying to interpret syntactically invalid code. It is thrown when the JavaScript engine encounters tokens or token order that does not conform to the syntax of the language when parsing code.

SyntaxError is a serializable object, so it can be cloned with structuredClone() or copied between Workers using postMessage() .

SyntaxError is a subclass of Error .

Constructor

Creates a new SyntaxError object.

Instance properties

Also inherits instance properties from its parent Error .

These properties are defined on SyntaxError.prototype and shared by all SyntaxError instances.

The constructor function that created the instance object. For SyntaxError instances, the initial value is the SyntaxError constructor.

Represents the name for the type of error. For SyntaxError.prototype.name , the initial value is «SyntaxError» .

Instance methods

Inherits instance methods from its parent Error .

Examples

Catching a SyntaxError

try  eval("hoo bar"); > catch (e)  console.log(e instanceof SyntaxError); // true console.log(e.message); console.log(e.name); // "SyntaxError" console.log(e.stack); // Stack of the error > 

Creating a SyntaxError

try  throw new SyntaxError("Hello"); > catch (e)  console.log(e instanceof SyntaxError); // true console.log(e.message); // "Hello" console.log(e.name); // "SyntaxError" console.log(e.stack); // Stack of the error > 

Specifications

Browser compatibility

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See also

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This page was last modified on May 26, 2023 by MDN contributors.

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How to check javascript code for syntax errors only from the command line?

Verifying syntax of a JavaScript code is crucial to identify any potential errors that could cause the program to fail or behave unexpected. This can be done using various tools and techniques, including text editors with built-in linting, integrated development environments (IDEs), or directly from the command line. In this answer, we will provide the different methods to check JavaScript code for syntax errors using only the command line.

Method 1: Using JavaScript Linter

One of the most popular ways to check JavaScript code for syntax errors from the command line is by using a JavaScript linter. A linter is a tool that analyzes your code for potential errors and provides suggestions for improving your code.

To use a JavaScript linter, you need to install it first. In this tutorial, we will be using ESLint as our linter.

Step 1: Install ESLint

To install ESLint, open your terminal and run the following command:

npm install eslint --save-dev

This will install ESLint as a development dependency in your project.

Step 2: Configure ESLint

After installing ESLint, you need to configure it by creating a .eslintrc file in the root of your project. This file contains the configuration options for ESLint.

To create the .eslintrc file, run the following command:

This command will prompt you with a series of questions to help you configure ESLint. You can choose the default options or customize them according to your needs.

Step 3: Run ESLint

Once you have configured ESLint, you can run it from the command line to check your JavaScript code for syntax errors.

To run ESLint, use the following command:

Replace your-file.js with the path to your JavaScript file.

ESLint will analyze your code and report any syntax errors it finds. It will also provide suggestions for improving your code.

Method 2: Using Node.js REPL

To check JavaScript code for syntax errors ONLY from the command line, you can use the Node.js REPL (Read-Eval-Print Loop) tool. Here are the steps to do it:

1. Open your terminal or command prompt and type node to start the Node.js REPL.
2. Type or paste your JavaScript code into the REPL prompt and press Enter.
3. If there is a syntax error in your code, the REPL will display an error message that describes the error and the line number where it occurred.

Here is an example of how to use the Node.js REPL to check for syntax errors:

$ node > const a = 10 undefined > if (a === 10)  . console.log("Hello, world!") . > Hello, world! undefined > if (a == 10  . console.log("Hello, world!") . > Thrown: if (a == 10  ^ SyntaxError: Unexpected token  at new Script (vm.js:85:7) at createScript (vm.js:266:10) at repl:1:1 at Script.runInThisContext (vm.js:120:18) at REPLServer.defaultEval (repl.js:433:29) at bound (domain.js:427:14) at REPLServer.runBound [as eval] (domain.js:440:12) at REPLServer.onLine (repl.js:760:10) at REPLServer.emit (events.js:315:20) at REPLServer.EventEmitter.emit (domain.js:482:12)

In this example, the first two lines of code are executed without errors, but the third line contains a syntax error (missing a closing parenthesis), so the REPL displays an error message with the line number and a description of the error.

You can also use the —check flag with the node command to check a JavaScript file for syntax errors. For example:

This will check the syntax of myfile.js and display any syntax errors in the console.

Note that the —check flag is only available in Node.js version 8.0.0 or later.

That’s it! With these simple steps, you can use the Node.js REPL to quickly check your JavaScript code for syntax errors from the command line.

Method 3: Using JSHint

To check JavaScript code for syntax errors only from the command line using JSHint, follow these steps:

1. Install Node.js on your computer.
2. Open your terminal or command prompt.
3. Install JSHint globally by running the following command:

1. Navigate to the directory where your JavaScript file is located.
2. Run the following command to check your JavaScript file for syntax errors:

// sample.js var x = 5 console.log(x)

In the terminal or command prompt, navigate to the directory where the sample.js file is located and run the following command:

sample.js: line 2, col 9, Missing semicolon. 1 error

In this example, JSHint detected a missing semicolon on line 2 of the sample.js file.

Note: JSHint has many configuration options to customize its behavior. For more information, refer to the JSHint documentation.

Method 4: Using ESLint

ESLint is a popular tool that helps to find and fix problems in your JavaScript code. It can be used to check for syntax errors from the command line. Here are the steps to do it:

Step 1: Install ESLint

To use ESLint, you need to install it first. You can do this using npm, the package manager for Node.js:

npm install eslint --save-dev

This will install ESLint locally in your project, and save it as a development dependency.

Step 2: Create a configuration file

ESLint needs a configuration file to know which rules to apply. You can create a configuration file manually, or use the init command to generate one for you:

This will ask you a series of questions to generate a configuration file based on your preferences.

Step 3: Run ESLint

Once you have a configuration file, you can run ESLint on your JavaScript code:

This will check your file for syntax errors and report them on the command line.

Step 4: Fix errors

If ESLint finds syntax errors, it will report them on the command line with a description of the problem and the line number where it occurred. You can then fix the errors and run ESLint again to make sure everything is correct.

Example code

Here is an example of running ESLint on a JavaScript file:

This will check the file index.js for syntax errors and report them on the command line.

// index.js const foo = 'bar' console.log(foo)

If there are no syntax errors, ESLint will not report anything. If there are errors, it will report them on the command line:

index.js 1:6 error 'foo' is assigned a value but never used no-unused-vars ✖ 1 problem (1 error, 0 warnings)

This error is caused by the variable foo being assigned a value but never used. To fix it, you can remove the line that assigns the value:

When you run ESLint again, it should not report any errors:

This will check the file index.js for syntax errors and report that there are no errors on the command line.

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JavaScript validator

JavaScript validator allows to test and validate JavaScript online, and find JavaScript errors easily and quickly. This JavaScript code checker tool highlights and allows to navigate between lines with syntax error. To check your code, you must copy and paste, drag and drop a .js file or directly type in the «JavaScript» online editor below, and click on «Check JavaScript syntax» button.

This tool also is a JavaScript fiddle, you can fill html, CSS and JavaScript editors in order to run your source directly in your browser. You can see the user guide to help you to use this JavaScript fiddle tool.

User guide

• First, Drag and drop your JavaScript file or copy / paste your JavaScript text directly into the editor above.
• Finally, you must click on «Check JavaScript syntax» button to display if there is an syntax error in your code.

To run and execute your JavaScript code in your browser, it is analogous to the check js syntax, except that you must click on «RUN fiddle» button.

To share your code, You must only click on «SHARE fiddle» button (And choice a password)!

JavaScript checker

JavaScript Validator Online tool allows to find syntax errors (lint). If a syntax error is detected, then the line in error is highlighted.

This linter allows to report stylistic errors, it can improve the quality of the code. It can also signal suspicious constructs, avoid mistakes, and potentially dangerous things.

JavaScript fiddle

You can test your JavaScript code online directly in your browser and share it!

You just have to share the URL of your fiddle in order to share it. Your JS fiddle will be accessible to everyone, do not save sensitive data! The password is only used to protect its modification.

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