What causes "Expected ':' after object key" in karatelabs/karate?

Insert the missing `:` between key and value in the JSON text.

Expected ':' after object key

Error message

Expected ':' after object key

What it means

After reading an object key, parseObject requires a `:` separator before the value. Missing separators, stray characters (e.g. `=` as in JS `var x = {...}` fragments), or a truncated key/value pair cause "Expected ':' after object key".

Solutions

Insert the missing `:` between key and value in the JSON text.
Generate JSON programmatically (serializer/library) instead of string concatenation.
Check for characters like `=` or `=>` that replaced the colon and fix them.

Example fix

// before
var x = JSON.parse('{"a" 1}');

// after
var x = JSON.parse('{"a": 1}');

Defensive patterns

Strategy: validation

Validate before calling

// detect missing colon between key and value: "key" followed by non-colon
if (/"\s*[^\s:]\s*[,}\d]/.test(input) && !/"\s*:/.test(input)) {
    throw new IllegalArgumentException('each key needs a colon before its value');
}

Try / catch

try {
    var obj = JSON.parse(input);
} catch (e) {
    if (('' + e).indexOf("Expected ':' after object key") >= 0) {
        console.log('a colon is missing between a key and its value');
    }
    throw e;
}

Prevention

Build JSON with a serializer instead of string concatenation.
After editing JSON by hand, re-check every `key value` pair for its colon.
Use an editor with JSON syntax highlighting to catch missing separators.

When it happens

Trigger: Parsing `{"a" 1}`, `{"a" = 1}`, or input where whitespace/newline corruption removed the colon between key and value; nested parseValue calls then re-enter parseObject and fail at the separator check.

Common situations: Editing JSON by hand and deleting a colon; string manipulation that splits/mangles key-value pairs; generating JSON via string concatenation instead of a serializer.

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.

Related errors

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/a415bcf781de0901. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsonParser.java:120

        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();
                skipWs();
                if (pos >= len || s.charAt(pos) != ':') {
                    throw syntaxError("Expected ':' after object key");
                }
                pos++;
                skipWs();
                Object value = parseValue();
                // RFC 8259: behavior of duplicate keys is unspecified; we keep
                // the last value (matches json-smart parseKeepingOrder).
                map.put(key, value);
                skipWs();
                if (pos >= len) {
                    throw syntaxError("Unexpected end of JSON input in object");
                }
                char c = s.charAt(pos);
                if (c == ',') {
                    pos++;
                    // trailing-comma is invalid per spec: the next iteration
                    // demands a string key, which rejects e.g. {"a":1,}.
                    continue;
                }

View on GitHub (pinned to a22eb90246)