JSON.parse: unexpected character at line 1 column 1 of the JSON data

What the error means

This is Firefox's wording for a JSON.parse failure. Line 1 column 1 means the very first character cannot start a JSON value. A value can only start with {, [, a double quote, a digit, a minus sign, or the first letter of true, false or null. So the text is usually not JSON at all: an HTML page, a PHP warning printed before the JSON, a plain-text error, the word undefined, or an invisible byte-order mark. Firefox counts lines and columns from 1, skips leading spaces and newlines, and reports the character it rejected.

Exact messages by runtime

Messages depend on the input and runtime version. Node and Python rows were reproduced locally. Other rows cite official documentation or source. Rows marked as fragments or templates preserve the source wording and are not complete exception transcripts. Browsers and libraries may append location details.

Causes you can reproduce

const body = '<br />\n<b>Warning</b>: Undefined variable $id';
JSON.parse(body);
// Firefox: JSON.parse: unexpected character at line 1 column 1 of the JSON data

JSON.parse('Internal Server Error');
// Node: Unexpected token 'I', "Internal S"... is not valid JSON

import { readFileSync, writeFileSync } from 'node:fs';
writeFileSync('bom.json', '\uFEFF{"a":1}');
JSON.parse(readFileSync('bom.json', 'utf8'));
// SyntaxError: Unexpected token ... is not valid JSON (the token is U+FEFF)

How to fix it

1. Look at the raw text. In DevTools, open the failing request in the Network panel and check its status, Content-Type and response body.
2. Make invisible characters visible by logging JSON.stringify(text.slice(0, 80)). A byte-order mark shows as \ufeff, and HTML or a PHP warning shows its first tag.
3. Use the first character: < means HTML, a letter means plain text or undefined, and \ufeff means a byte-order mark. An empty body gives unexpected end of data instead.
4. Fix the source: the URL, the session, the server error or the PHP warning. If the column is greater than 1, the text starts as JSON and breaks later, so match the rest of the message to the related guides.

async function readJson(response) {
  const text = await response.text();
  if (!response.ok) {
    throw new Error(`HTTP ${response.status}: ${JSON.stringify(text.slice(0, 80))}`);
  }
  try {
    return JSON.parse(text);
  } catch (error) {
    throw new Error(`Expected JSON, got ${JSON.stringify(text.slice(0, 80))}`, { cause: error });
  }
}

import { readFileSync } from 'node:fs';
const raw = readFileSync('bom.json', 'utf8');
const text = raw.charCodeAt(0) === 0xfeff ? raw.slice(1) : raw;
console.log(JSON.parse(text)); // { a: 1 }

Find the exact position with Tool Kumar

1. Copy the exact input text, including its original whitespace. For HTTP failures, copy the raw response body.
2. Open the Tool Kumar JSON validator and paste the text. The document stays in your browser.
3. Read the line and column in the error or fixable-issue banner. For an invalid document, select Go to error to move the caret to that location.
4. Inspect the preceding token as well as the marked character. A parser marks where it discovered the problem; the missing quote, separator or value can start earlier.

Python character offsets are zero-based, while its displayed lines and columns are one-based. V8 string positions count UTF-16 code units; Go offsets count bytes read. Unicode text can make these offsets differ. Use the original text and its line/column context rather than copying an offset between runtimes.

Tool Kumar can classify some input as fixable loose JSON. Review a proposed change before using the JSON repair tool, then validate the result against your API contract. For empty input, inspect the producer or response because there is no character to select.

Frequently asked questions

Related errors and tools

• Unexpected token '<' / DOCTYPE is not valid JSON
• Unexpected end of JSON input: empty or truncated data
• "undefined" or "[object Object]" is not valid JSON
• PHP json_decode returns null: Syntax error and other causes
• Expecting ',' delimiter / Expected ',' or '}' after property value
• All JSON error guides
• JSON syntax reference

Standards and diagnostic sources

RFC 8259 and ECMA-404 define JSON syntax. Diagnostic wording belongs to the parser. The table links each cited message to its source.

• MDN JSON.parse error reference
• Python json documentation and JSONDecodeError attributes
• WebKit source for the JSON Parse error prefix