JavaScript

Unexpected token (SyntaxError)

The JavaScript (or JSON) parser hit a character it didn't expect — usually a typo in the syntax.

What it looks like

Uncaught SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

What does "Unexpected token (SyntaxError)" mean?

Thrown either by the JavaScript parser (a real syntax error in a file) or by JSON.parse and response.json() (the text was not JSON). An unexpected < almost always means an HTML page — often an error or login page — arrived where JSON was expected.

What causes "Unexpected token (SyntaxError)"?

• A missing/extra bracket, comma, or quote.
• Pasting HTML/an error page where JSON was expected.
• Using new syntax an old engine doesn't support.

How do I confirm the cause?

1. If the message mentions JSON, open the request in the Network tab and look at the raw response — it is probably HTML.
2. For a code error, go to the line and column given and check the characters just before it: a missing bracket, comma or quote.
3. If the code runs in some browsers and not others, check for syntax newer than the oldest browser you support.

How do I fix "Unexpected token (SyntaxError)"?

• Look at the line/column the error points to and just before it.
• Run it through a linter or JSON validator.
• Check the response was actually JSON, not an HTML error page.

How do I stop it happening again?

• Check response.ok and the Content-Type before calling response.json().
• Run a linter or formatter in your editor so syntax errors show up before you save.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.

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