karatelabs/karate · error · RuntimeException

fromJson() needs one argument: a JSON string

Error message

fromJson() needs one argument: a JSON string

What it means

Karate's fromJson() JS utility parses a JSON string into a Java object graph. It requires one string argument; calling it with no arguments throws this error. Non-string arguments are tolerated (returned as already-parsed values), so only the missing-argument case triggers this.

Solutions

1. Pass the JSON string: fromJson('{"a":1}')
2. Check the variable feeding fromJson is actually defined and a string
3. If the value is already an object, fromJson can be dropped entirely

Example fix

// before
var obj = karate.fromJson();
// after
var obj = karate.fromJson('{"a":1}');

if (typeof json !== 'string') throw new Error('fromJson expects a JSON string');

function canParseJson(v) { return typeof v === 'string' && v.trim().length > 0; }

try { obj = karate.fromJson(str); } catch (e) { karate.log('fromJson failed: ' + e.message); }

When it happens

Trigger: Calling fromJson() with zero arguments, e.g. fromJson(), or passing undefined where a JSON string was expected.

Common situations: Variable holding the JSON string is undefined/null due to a failed earlier step; forgetting the argument when converting from Json.parse style code.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• eval() needs one argument
• expect() needs at least one argument
• remove() needs two arguments: variable name and path
• append() needs at least two arguments
• appendTo() needs at least two arguments: list and item(s)

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/3b8890962c8b5620. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJsUtils.java:263

            if (collection instanceof List<?> list) {
                for (int i = 0; i < list.size(); i++) {
                    fn.call(null, new Object[]{list.get(i), i});
                }
            } else if (collection instanceof Map) {
                Map<String, Object> map = (Map<String, Object>) collection;
                int i = 0;
                for (Map.Entry<String, Object> entry : map.entrySet()) {
                    fn.call(null, new Object[]{entry.getKey(), entry.getValue(), i++});
                }
            }
            return null;
        };
    }

    static JavaInvokable fromJson() {
        return args -> {
            if (args.length < 1) {
                throw new RuntimeException("fromJson() needs one argument: a JSON string");
            }
            Object arg = args[0];
            if (arg instanceof String str) {
                return Json.of(str).value();
            } else {
                // Already parsed, just return it
                return arg;
            }
        };
    }

    static JavaInvokable jsonPath() {
        return args -> {
            if (args.length < 2) {
                throw new RuntimeException("jsonPath() needs two arguments: object and path");
            }
            Object json = args[0];
            if (json instanceof Node node) {

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