karatelabs/karate · error

Unexpected token ' ' in JSON

Error message

Unexpected token '${c}' in JSON

What it means

The JSON parser encountered a character that cannot start a JSON value where one was required. After switching on `{`, `[`, `"`, and literal starts, the default branch allows only `-` or digits for numbers; anything else triggers "Unexpected token '<c>' in JSON". This mirrors JSON.parse's own error style.

Solutions

1. Quote strings with double quotes and use only JSON literals (null, true, false, numbers) in the input.
2. Strip any non-JSON prefix/suffix (e.g. HTML, BOM, log lines) before parsing.
3. Validate the payload with a linter or JSON.parse in a test to confirm it is strict JSON.

Example fix

// before
JSON.parse("{'name': 'a'}");

// after
JSON.parse('{"name": "a"}');

// cheap pre-check for obviously non-JSON starts
var t = input.trim();
if (!(t.startsWith('{') || t.startsWith('[') || t.startsWith('"') || /^-?[0-9tfn]/.test(t))) {
    throw new IllegalArgumentException('input does not look like JSON: ' + t.substring(0, 20));
}

try {
    var obj = JSON.parse(input);
} catch (e) {
    if (('' + e).indexOf('Unexpected token') >= 0) {
        console.log('non-JSON content near start of: ' + input.substring(0, 40));
    }
    throw e;
}

When it happens

Trigger: Parsing input like `"undefined"`, `"{'a':1}"` (single quotes), `"NaN"`, or stray text before/after a JSON document, where the offending character reaches parseValue's default branch.

Common situations: Passing JS literals (undefined, NaN, single-quoted objects) where strict JSON is required; response bodies that contain HTML error pages; copy-pasted JSON with smart quotes or BOM/leading garbage.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Expected string key in object
• Expected ':' after object key
• Unexpected end of JSON input in object
• Unexpected end of JSON input in array
• Invalid number: bare '-'

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/f00acf7a343cbdbc. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsonParser.java:99

            }
            char c = s.charAt(pos);
            switch (c) {
                case '{':
                    return parseObject();
                case '[':
                    return parseArray();
                case '"':
                    return parseString();
                case 't':
                case 'f':
                    return parseBoolean();
                case 'n':
                    return parseNull();
                default:
                    if (c == '-' || (c >= '0' && c <= '9')) {
                        return parseNumber();
                    }
                    throw syntaxError("Unexpected token '" + c + "' in JSON");
            }
        }

        private Map<String, Object> parseObject() {
            // we know s.charAt(pos) == '{'
            pos++;
            Map<String, Object> map = new LinkedHashMap<>();
            skipWs();
            if (pos < len && s.charAt(pos) == '}') {
                pos++;
                return map;
            }
            while (true) {
                skipWs();
                if (pos >= len || s.charAt(pos) != '"') {
                    throw syntaxError("Expected string key in object");
                }
                String key = parseString();

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