---
title: Custom Functions
description: "Call TypeScript functions from YAML with the call field, passing page/request/testContext to read and write runtime variables."
---

# Custom Functions

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Call TypeScript functions from YAML using the `call` field with `file#export` syntax:

```yaml
statements:
  - intent: Seed test data
    call: "../helpers/seed.ts#create_test_user"
    args: [page, testContext, "test@example.com"]
```

Inside your function, use `testContext` to read and write runtime variables:

```ts
// helpers/seed.ts
export async function create_test_user(page, testContext, email: string) {
  // Read a variable
  const baseUrl = testContext.BASE_URL;

  // Write a variable (available to subsequent YAML steps as {{userId}})
  testContext.userId = "user-123";
}
```

Each value in `args` maps directly to a parameter in the function signature. System objects (`page`, `request`, `testContext`) are passed as-is, strings are quoted, and numbers stay numeric.