---
sidebar_position: 50
sidebar_label: Javascript
description: Build sophisticated JavaScript validators for LLM outputs with async operations, scoring logic, and comprehensive error handling
---

# Javascript assertions

The `javascript` [assertion](/docs/configuration/expected-outputs) allows you to provide a custom JavaScript function to validate the LLM output.

A variable named `output` is injected into the context. The function should return `true` if the output passes the assertion, and `false` otherwise. If the function returns a number, it will be treated as a score.

You can use any valid JavaScript code in your function. The output of the LLM is provided as the `output` variable:

```yaml
assert:
  - type: javascript
    value: "output.includes('Hello, World!')"
```

In the example above, the `javascript` assertion checks if the output includes the string "Hello, World!". If it does, the assertion passes and a score of 1 is recorded. If it doesn't, the assertion fails and a score of 0 is returned.

Single-line assertions may begin with `const`, `let`, or `var` declarations; the final expression is returned automatically. Semicolons and quotes inside `/* ... */` or trailing `// ...` comments do not affect that expression. For example, `const n = output.length; /* characters; not words */ n > 5` returns whether the output has more than five characters. Optional chaining also works with keyword-named properties, such as `const data = JSON.parse(output); data?.default / 2 === 2`. Async functions can use regular expressions after `await`, for example `const check = async () => await /[a-z]+/.test(output); check()`. Generator functions and methods, including async generators, can also use regular expressions after `yield`: `const check = async function* () { yield /[a-z]+/.test(output); }; check().next().then(result => result.value)`.

For longer assertions, use [multiline functions](#multiline-functions) with an explicit `return`.

If you want to return a custom score, your function should return a number. For example:

```yaml
assert:
  - type: javascript
    value: Math.log(output.length) * 10
    threshold: 0.5 # any value above 0.5 will pass
```

In the example above, the longer the output, the higher the score.

If your function throws an error, the assertion will fail and the error message will be included in the reason for the failure. For example:

```yaml
assert:
  - type: javascript
    value: |
      if (errorCase) {
        throw new Error('This is an error');
      }
      return {
        pass: false,
        score: 0,
        reason: 'Assertion failed',
      };
```

## Handling objects

For string expressions and `file://` scripts, `output` keeps its type after any provider, test, or assertion [transforms](/docs/configuration/guide#transforming-outputs). If it is an object or array (such as tool/function calls), access it directly:

```yaml
assert:
  - type: javascript
    value: output[0].function.name === 'get_current_weather'
```

JSON text remains a string; use `JSON.parse(output)` to parse it. [Inline function assertions](#inline-assertions) always receive a string, with objects serialized as JSON.

## Return type

The return value of your Javascript function can be a boolean, number, or a `GradingResult`:

```typescript
type JavascriptAssertionResult = boolean | number | GradingResult;

// Used for more complex results
interface GradingResult {
  pass: boolean;
  score: number;
  reason: string;
  namedScores?: Record<string, number> | null;
  namedScoreWeights?: Record<string, number> | null;
  componentResults?: GradingResult[] | null;
}
```

If `componentResults` is set, a table of assertion details will be shown in the test output modal in the Eval view.

Numeric returns, `score`, and all values in `namedScores` and `namedScoreWeights` must be finite, including in nested `componentResults`. `NaN` or infinity fails the assertion; finite scores outside 0–1 are accepted. `pass: false` still fails when `reason` is empty.

A `componentResults` array must contain a valid grading result at every index; sparse arrays are rejected.

In results from custom graders and scoring functions, a named score that is a boolean, `null`, or a numeric string is recorded as a number: `true` is 1, and `false` and `null` are 0. A nested component result may omit `reason` and `score`; an omitted score is 1 when `pass` is true and 0 otherwise.

## Multiline functions

Javascript assertions support multiline strings:

```yaml
assert:
  - type: javascript
    value: |
      // Insert your scoring logic here...
      if (output === 'Expected output') {
        return {
          pass: true,
          score: 0.5,
          reason: 'Output matches the expected value',
        };
      }
      return {
        pass: false,
        score: 0,
        reason: 'Assertion failed',
      };
```

## Using test context

The `context` variable contains information about the test case and execution environment:

```ts
interface TraceSpan {
  spanId: string;
  parentSpanId?: string;
  name: string;
  startTime: number; // Unix timestamp in milliseconds
  endTime?: number; // Unix timestamp in milliseconds
  attributes?: Record<string, any>;
  statusCode?: number;
  statusMessage?: string;
}

interface TraceData {
  traceId: string;
  spans: TraceSpan[];
}

interface AssertionValueFunctionContext {
  // Raw prompt sent to LLM
  prompt: string | undefined;

  // Test case variables
  vars: Record<string, string | object>;

  // The complete test case
  test: AtomicTestCase;

  // Log probabilities from the LLM response, if available
  logProbs: number[] | undefined;

  // Configuration passed to the assertion
  config?: Record<string, any>;

  // The provider that generated the response
  provider: ApiProvider | undefined;

  // The complete provider response
  providerResponse: ProviderResponse | undefined;

  // OpenTelemetry trace data (when tracing is enabled)
  trace?: TraceData;

  // Shortcut to providerResponse?.metadata (provider-specific fields)
  metadata?: Record<string, any>;
}
```

For example, if the test case has a var `example`, access it in your JavaScript function like this:

```yaml
tests:
  - description: 'Test with context'
    vars:
      example: 'Example text'
    assert:
      - type: javascript
        value: 'output.includes(context.vars.example)'
```

You can also use the `context` variable to perform more complex checks. For example, you could check if the output is longer than a certain length defined in your test case variables:

```yaml
tests:
  - description: 'Test with context'
    vars:
      min_length: 10
    assert:
      - type: javascript
        value: 'output.length >= context.vars.min_length'
```

## Passing assertion-specific parameters

If you want to reuse the same JavaScript assertion with different parameters in a single test case, prefer assertion-level `config` over test `vars`. Test vars are shared across all assertions and appear as report columns, while `config` stays attached to one assertion and is available as `context.config`.

```yaml
tests:
  - description: 'Reuse one assertion script with two thresholds'
    vars:
      topic: 'bananas'
    assert:
      - type: javascript
        value: file://assertions/min-length.js
        config:
          minLength: 5
      - type: javascript
        value: file://assertions/min-length.js
        config:
          minLength: 20
```

```js
module.exports = (output, context) => {
  return output.length >= context.config.minLength;
};
```

## External script

To reference an external file, use the `file://` prefix:

```yaml
assert:
  - type: javascript
    value: file://relative/path/to/script.js
    config:
      maximumOutputSize: 10
```

You can specify a particular function to use by appending it after a colon:

```yaml
assert:
  - type: javascript
    value: file://relative/path/to/script.js:customFunction
```

The JavaScript file must export an assertion function. Here are examples:

```js
// Default export
module.exports = (output, context) => {
  return output.length > 10;
};
```

```js
// Named exports
module.exports.customFunction = (output, context) => {
  return output.includes('specific text');
};
```

Here's an example using configuration data defined in the assertion's YAML file:

```js
module.exports = (output, context) => {
  return output.length <= context.config.maximumOutputSize;
};
```

Here's a more complex example that uses an async function to hit an external validation service:

```js
const VALIDATION_ENDPOINT = 'https://example.com/api/validate';

module.exports = async (output, context) => {
  const response = await fetch(VALIDATION_ENDPOINT, {
    method: 'POST',
    headers: {
      'Content-Type': 'text/plain',
    },
    body: output,
  });

  return response.json();
};
```

You can also return complete [`GradingResult`](/docs/configuration/reference/#gradingresult) objects. For example:

```js
module.exports = (output, context) => {
  console.log('Prompt:', context.prompt);
  console.log('Vars', context.vars.topic);

  // You can return a bool...
  // return output.toLowerCase().includes('bananas');

  // A score (where 0 = Fail)...
  // return 0.5;

  // Or an entire grading result, which can be simple...
  let result = {
    pass: output.toLowerCase().includes('bananas'),
    score: 0.5,
    reason: 'Contains banana',
  };

  // Or include nested assertions...
  result = {
    pass: true,
    score: 0.75,
    reason: 'Looks good to me',
    componentResults: [
      {
        pass: output.toLowerCase().includes('bananas'),
        score: 0.5,
        reason: 'Contains banana',
        namedScores: {
          'Uses banana': 1.0,
        },
      },
      {
        pass: output.toLowerCase().includes('yellow'),
        score: 0.5,
        reason: 'Contains yellow',
        namedScores: {
          Yellowish: 0.66,
        },
      },
    ],
  };

  return result;
};
```

## Inline assertions

If you are using promptfoo as a JS package, you can build your assertion inline:

```js
{
  type:"javascript",
  value: (output, context) => {
    return output.includes("specific text");
  }
}
```

Here `output` is always a string, so if your [custom response parser](/docs/providers/http/#function-parser) returned an object, use `JSON.parse(output)` to convert it back.

## Using trace data

When [tracing is enabled](/docs/tracing/), OpenTelemetry trace data is available in the `context.trace` object. This allows you to write assertions based on the execution flow:

```js
module.exports = (output, context) => {
  // Check if trace data is available
  if (!context.trace) {
    // Tracing not enabled, skip trace-based checks
    return true;
  }

  const { spans } = context.trace;

  // Example: Check for errors in any span
  const errorSpans = spans.filter((s) => s.statusCode >= 400);
  if (errorSpans.length > 0) {
    return {
      pass: false,
      score: 0,
      reason: `Found ${errorSpans.length} error spans`,
    };
  }

  // Example: Calculate total trace duration
  if (spans.length > 0) {
    const duration =
      Math.max(...spans.map((s) => s.endTime || 0)) - Math.min(...spans.map((s) => s.startTime));
    if (duration > 5000) {
      // 5 seconds
      return {
        pass: false,
        score: 0,
        reason: `Trace took too long: ${duration}ms`,
      };
    }
  }

  // Example: Check for specific operations
  const apiCalls = spans.filter((s) => s.name.toLowerCase().includes('http'));
  if (apiCalls.length > 10) {
    return {
      pass: false,
      score: 0,
      reason: `Too many API calls: ${apiCalls.length}`,
    };
  }

  return true;
};
```

Example YAML configuration:

```yaml
tests:
  - vars:
      query: "What's the weather?"
    assert:
      - type: javascript
        value: |
          // Ensure retrieval happened before response generation
          if (context.trace) {
            const retrievalSpan = context.trace.spans.find(s => s.name.includes('retrieval'));
            const generationSpan = context.trace.spans.find(s => s.name.includes('generation'));
            
            if (retrievalSpan && generationSpan) {
              return retrievalSpan.startTime < generationSpan.startTime;
            }
          }
          return true;
```

Additional examples:

```js
// Check span hierarchy depth
const MAX_ALLOWED_DEPTH = 1000;
const maxDepth = (spans, parentId = null, depth = 0) => {
  if (depth > MAX_ALLOWED_DEPTH) {
    throw new Error('Span hierarchy too deep');
  }
  const children = spans.filter((s) => s.parentSpanId === parentId);
  if (children.length === 0) return depth;
  return Math.max(...children.map((c) => maxDepth(spans, c.spanId, depth + 1)));
};

if (context.trace && maxDepth(context.trace.spans) > 5) {
  return {
    pass: false,
    score: 0,
    reason: 'Call stack too deep',
  };
}
```

### ES modules

ES modules are supported, but must have a `.mjs` file extension. Alternatively, if you are transpiling Javascript or Typescript, we recommend pointing promptfoo to the transpiled plain Javascript output.

## Negation

Use `not-javascript` to invert the final pass/fail result while preserving the returned score. Numeric scores are still compared against `threshold` before the result is inverted:

```yaml
assert:
  - type: not-javascript
    value: output.includes('error')
```

## Other assertion types

For more info on assertions, see [Test assertions](/docs/configuration/expected-outputs).