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SyntaxError: Unexpected Token

SyntaxError: Unexpected Token

Shivan M.

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The Problem

The SyntaxError: Unexpected token error in JavaScript indicates that a specific language construct was expected, but something else was found. Why might I encounter this error?

The Solution

This usually happens because of a typo or, in some cases, because a server or third-party service returned an unexpected response.

Fixing Typos

If this error occurs because of a typo, it is a good idea to look for code that is supposed to contain a specific language construct, for example, opening and closing braces, quotation marks, and semicolons.

In the example below, the for loop has a comma preceding the semicolon. In this case, JavaScript expected a semicolon but found the comma instead, resulting in an error:

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for (let i = 0; i < 5,; ++i) {
  console.log(i);
}

Checking Server Responses

You might encounter this error with the following message:

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Syntax error: unexpected token <

In this case, it is likely that the cause of the error is a server response that is expected to be a JSON value, but has instead returned HTML.

An instance where this might occur is when a server returns a 404 page with an HTTP 200 response code.

In this scenario, the server responded with an HTML page including a 404 response, but the response status code was 200. If your code parses this response as JSON, you will encounter the above error.

This can also occur if a backend server responds with an error that is HTML formatted, with a response code that your application treats as valid. The following code example illustrates how this might happen:

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async function getPosts() {
    const posts = await fetch('/api/posts');
    if (response.status === 200) {
    // if the server responded with a 200 OK with an HTML formatted error, this will raise an exception
    const postsData = JSON.parse(response.data)
  }
}

Additional Resources

  • YoutubeHow Sentry.io saved me from disaster
  • ResourcesImprove Web Browser Performance - Find the JavaScript code causing slowdowns
  • SentryJavascript Error Monitoring & Tracing
  • ResourcesJavaScript Frontend Error Monitoring 101
  • Syntax.fmListen to the Syntax Podcast
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