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transform.script runs a typed TypeScript function in a secure WebAssembly (QuickJS) sandbox. Reach for it whenever a transformation is deterministic but too involved for a LiquidJS template, array map/filter/reduce, multi-step calculations, conditional branching, date arithmetic, or regex parsing.

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When to use Script vs LiquidJS

Use caseTool
Simple field access, basic filters, simple mathLiquidJS ({{ ... }})
Array reduce/map/filter, multi-step calculationsScript
Conditional branching, date math, string parsingScript
A good rule: if the logic spans more than two or three lines, use a script.

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The contract

The function must be named script. Its parameters come from the inputs map (each key becomes a parameter, in declaration order), and its return-type annotation is this step’s output schema. There is no separate outputSchema: field.
  • Parameter names and order must equal Object.keys(inputs). Enforced at push time.
  • The : R return annotation is mandatory and is derived to JSON Schema, so downstream steps autocomplete against it. A too-loose annotation (such as unknown) leaves downstream field references unresolvable.
  • No async, import(), or require(), the sandbox has no module loader and the worker calls the function synchronously.

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Example

- name: calculate-totals
  type: transform.script
  with:
    inputs:
      items: '{{ steps.extract.output.lineItems }}'
      taxRate: '{{ input.taxRate }}'
    function: |
      function script(
        items: { price: number; quantity: number }[],
        taxRate: number,
      ): { subtotal: number; tax: number; total: number } {
        const subtotal = items.reduce((sum, item) => sum + item.price * item.quantity, 0);
        const tax = subtotal * taxRate;
        return { subtotal, tax, total: subtotal + tax };
      }
Here { subtotal, tax, total } is the output schema, so a later step can read {{ steps.calculate-totals.output.total }}.

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Configuration

Configuration goes inside the step’s with: block.
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inputs
record<string, string>
Named inputs mapped from template expressions. Keys become the function parameter list in declaration order: inputs: &#123; items, taxRate } ⇒ function script(items: …, taxRate: …): R &#123; … }.
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function
string
required
TypeScript function declaration. Must be function script(args): R &#123; … } where the parameter list equals Object.keys(inputs) in order and R is a return type annotation. The annotation IS this step’s output schema.
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timeout
number
default:"5000"
Max execution time in milliseconds (default: 5000)
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memoryLimit
number
default:"10485760"
Max memory in bytes (default: 10MB)

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Output

Returns unknown. Value returned from script. Validated at runtime against the JSON Schema derived from the function’s return type annotation.