Javascript assertions

The javascript assertion 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:

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.

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

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:

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

Return type

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

type JavascriptAssertionResult = boolean | number | GradingResult;

// Used for more complex results
interface GradingResult {
  pass: boolean;
  score: number;
  reason: string;
  componentResults?: GradingResult[];
}

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

Multiline functions

Javascript assertions support multiline strings:

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

Using test context

The context variable contains the prompt and test case variables:

interface AssertContext {
  // Raw prompt sent to LLM
  prompt: string;

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

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

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:

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

External script

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

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

The Javascript file must export an assertion function. Here's an example:

module.exports = (output, context) => {
  return output.length > 10;
};

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

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

async function evaluate(modelResponse) {
  try {
    const response = await fetch(VALIDATION_ENDPOINT, {
      method: 'POST',
      headers: {
        'Content-Type': 'text/plain',
      },
      body: modelResponse,
    });

    const data = await response.json();
    return data;
  } catch (error) {
    throw error;
  }
}

async function main(output, context) {
  const success = await evaluate(output);
  console.log(`success: ${testResult}`);
  return success;
}

module.exports = main;

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.

Other assertion types

For more info on assertions, see Test assertions.