Node.js — SyntaxError: Unexpected token caught at startup

Locate syntax errors quickly by reading the reported position and isolating the file from the build output.

beginnerdebuggingnodesyntax

Symptoms

SyntaxError: Unexpected token < in JSON at position 0
SyntaxError: Unexpected token }
The error appears only after a build or deploy
The failing file runs in the editor but not under node

Possible causes

The file is HTML or an error page parsed as JavaScript or JSON
An unbalanced brace, bracket or quote in the source
A build artifact is stale or partially written
A JSON response returns an HTML login page

Diagnosis steps

Read the full error with the stack

node --stack-trace-limit=20 file.js

Interpretation

The frame above the error names the file and line, which is often enough to see the offending token.

Validate JSON separately when relevant

node -e "JSON.parse(require('fs').readFileSync('config.json','utf8'))"

Interpretation

Unexpected token < at position 0 almost always means HTML was parsed as JSON, such as a redirected request.

Rebuild to replace stale artifacts

rm -rf dist && npm run build

Interpretation

A half-written or outdated bundle can contain invalid syntax that no longer exists in the source.

Root cause

The runtime parsed text that was not valid JavaScript or JSON at the reported position, either because the source has a syntax error or because non-code content was parsed as code.

Fix

Fix the syntax error in the source, validate JSON before parsing, and rebuild so artifacts reflect the current source. For Unexpected token <, treat the response as an error page rather than data.

Verify the fix

Run the file again and confirm it loads, and rerun the parse on the actual data to confirm the content is now valid.

Symptom walkthrough

Node reports SyntaxError: Unexpected token with a position or a token that does not belong, and the file may have run fine in a tool with looser parsing. When the message is Unexpected token < in JSON at position 0, the parser almost certainly received an HTML page rather than JSON, often because an authenticated API returned a login screen.

Fix in practice

Read the reported line and fix the actual syntax error, or, for JSON failures, validate the response and treat non-JSON as an error rather than data. When the error only appears after a build, rebuild from clean to rule out stale artifacts that no longer match source.

Verification

Run the file again and confirm it loads without the syntax error, and parse the real payload to confirm the content is valid. Both checks together prove the parse path is healthy.

Diagnostic commands

Check the behavior with commands like these:

node --stack-trace-limit=20 file.js
node -e "JSON.parse(require('fs').readFileSync('config.json','utf8'))"
rm -rf dist && npm run build