Dangerous JS — Calculations

A calculation is a JavaScript expression attached to a field definition. It is evaluated after every response change and writes the computed result back into that field.

Calculations require the dual opt-in gate — both the host allowDangerousJS prop and the schema dangerouslyAllowJS: true flag must be set.

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Schema

Add a calculation string to any field definition:

{
  "id": "bmi",
  "fieldType": "text",
  "question": "BMI",
  "calculation": "Math.round((responses['weight'] / Math.pow(responses['height'] / 100, 2)) * 10) / 10"
}

The calculation is a JavaScript expression (not a statement). It is wrapped internally as:

new Function('responses', 'return ' + calculation)(data);

So the string must be a returnable expression:

// ✅ Valid — single expression
"responses['systolic'] - responses['diastolic']";

// ✅ Valid — ternary
"responses['age'] >= 18 ? 'Adult' : 'Minor'";

// ✅ Valid — IIFE for multi-step logic
"(() => { const d = new Date(responses['last-encounter']); d.setMonth(d.getMonth() + 6); return d.toISOString().slice(0, 10); })()";

// ❌ Invalid — statement syntax
"const x = responses['a']; return x + 1;";

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Expression Context

Every calculation receives two arguments:

responses

A flat Record<string, unknown> where each key is a field ID mapped to its normalized value:

Field type	responses['id'] value
text / longtext	Number if parseable, otherwise raw string
radio / dropdown	Option score (if set), otherwise option value string
check / multiselectdropdown	Array of selected values (or scores if any are set)
rating / slider	Option score or numeric value
boolean	Option value string ('Yes' / 'No' by default)
All others	Raw answer string

Unanswered fields return an empty string ''.

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Behavior

• Calculations run after every setResponse call.
• All fields with a calculation are re-evaluated in a single pass after each response update.
• Results are stored as { answer: String(result) }.
• If the calculation throws or returns null / undefined, the field's response is left unchanged.
• Calculated fields can be manually edited but may be overwritten on the next response change.
• Calculation writes do not recursively trigger another calculation pass.

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Examples

BMI from weight and height

{
  "id": "bmi",
  "fieldType": "text",
  "question": "BMI",
  "calculation": "Math.round((responses['weight'] / Math.pow(responses['height'] / 100, 2)) * 10) / 10"
}

Suggested follow-up date (6 months out)

{
  "id": "follow-up",
  "fieldType": "text",
  "question": "Suggested Follow-up Date",
  "calculation": "(() => { const d = new Date(responses['last-encounter']); d.setMonth(d.getMonth() + 6); return d.toISOString().slice(0, 10); })()"
}

Age from date of birth

{
  "id": "calculated-age",
  "fieldType": "text",
  "question": "Age",
  "calculation": "(() => { const dob = new Date(responses['dob']); if (!dob || isNaN(dob)) return ''; const today = new Date(); let age = today.getFullYear() - dob.getFullYear(); if (today < new Date(today.getFullYear(), dob.getMonth(), dob.getDate())) age--; return age; })()"
}

Conditional label from another field

{
  "id": "risk-label",
  "fieldType": "display",
  "question": "Risk Level",
  "calculation": "responses['total-risk-score'] >= 8 ? 'High Risk' : responses['total-risk-score'] >= 4 ? 'Moderate Risk' : 'Low Risk'"
}

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Legacy Migration Reference

The following legacy WebChart/EH functions have equivalents using responses and context:

Legacy function	eSheet equivalent
observationValueByName("field")	responses['field-id'] (within-form)

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Builder UI

When allowDangerousJS={true} is passed to <EsheetBuilder> and the loaded schema has dangerouslyAllowJS: true, the field editor shows a Calculation (JS) textarea where the expression can be entered directly.