Expression System

eSheet includes an expression evaluation system used in conditional logic and display field interpolation. Expressions can reference field values and perform arithmetic/logical operations.

Syntax

Field References

Reference any field's current answer value using curly braces:

{fieldId}

This resolves to:

• Text fields -> the answer string
• Selection fields -> the selected option's value string
• Multi-select fields -> the array of selected options
• Numeric text fields -> the numeric value (auto-parsed)

Computed Expressions

Wrap arithmetic or logical expressions in angle brackets:

<{fieldA} + {fieldB}>

Supports standard JavaScript-like operators:

• Arithmetic: +, -, *, /, %
• Comparison: >, >=, <, <=, ==, !=, ===, !==
• Logical: &&, ||, !
• Grouping: (, )
• Property access: .length, .count

Use in Display Fields

Display fields support inline expression interpolation in their content property:

id: summary
fieldType: display
content: |-
  # Summary

  Patient: *{name}*
  Age: *{age}*

  BMI: *<{weight} / (({height}/100) * ({height}/100))>*

  Total symptoms: *<{symptoms}.length>*

Markdown + Expressions

Display fields combine a Markdown-like syntax with expression interpolation:

Syntax	Renders as
*text*	bold
-text-	italic
_text_	underline
~text~	strikethrough
- item	bullet list
# Heading	heading
{fieldId}	field value
<expression>	computed result

Examples

Simple value display:

Your name is: *{name}*

Arithmetic:

Total: *<{price} * {quantity}>*

Conditional text (expression in condition rule, not in display):

rules:
  - effect: visible
    logic: AND
    conditions:
      - conditionType: expression
        expression: '{age} >= 18'

Use in Conditional Logic

Expression conditions in rules allow complex logic beyond simple field comparisons:

conditionType: expression
expression: '{weight} > 0 && {height} > 0'

Expression Condition Examples

Expression	Description
{age} >= 18	Age is 18 or older
{weight} > 0 && {height} > 0	Both weight and height are filled
{score} >= 80	Score is 80 or above
{symptoms}.length >= 3	At least 3 symptoms selected
{systolic} > 140 || {diastolic} > 90	Either blood pressure reading is high

Validation

Use isExpressionValid() from @esheet/core to validate expression syntax:

import { isExpressionValid } from '@esheet/core';

isExpressionValid('{age} >= 18'); // true
isExpressionValid('{weight} / {height}'); // true
isExpressionValid('{incomplete expression'); // false

How Evaluation Works

1. Field references ({fieldId}) are resolved against the current FormStore responses
2. The expression string is parsed and evaluated with resolved values
3. For display fields, the result is converted to a string and inserted inline
4. For conditional rules, the result is coerced to a boolean

warning

Expressions are evaluated client-side. Do not use them for security-sensitive logic. Always validate on the server.