handle-modus-checkbox-value-bug

name	handle-modus-checkbox-value-bug
description	Apply the critical checkbox value inversion workaround when working with ModusCheckbox components

Handle Modus Checkbox Value Bug

Apply the critical value inversion workaround for ModusWcCheckbox components.

Critical Bug

The ModusWcCheckbox web component has a value inversion bug where the value property returns the opposite of the actual checked state:

• When checkbox is checked: target.value returns false
• When checkbox is unchecked: target.value returns true

This is the opposite of what developers expect.

When to Use

Use this skill when:

• Implementing checkbox functionality with ModusCheckbox
• Fixing checkbox-related bugs
• Handling checkbox value changes
• Creating forms with checkboxes

Reference Implementation

See src/components/ModusCheckbox.tsx:116-130 for the complete workaround implementation.

The Fix

Always invert the value when reading from checkbox events:

const handleValueChange = (event: Event) => {
  const customEvent = event as CustomEvent<InputEvent>;
  
  // 🚨 CRITICAL: Handle the value inversion bug
  const rawValue = (customEvent.target as HTMLModusWcCheckboxElement).value;
  const actualValue = !rawValue; // ✅ CORRECT: Invert the value
  
  // Use actualValue for your logic
  setChecked(actualValue);
};

Complete Pattern

import { useEffect, useRef } from "react";
import { ModusWcCheckbox } from "@trimble-oss/moduswebcomponents-react";

export default function ModusCheckbox({
  value = false,
  onValueChange,
}: {
  value?: boolean;
  onValueChange?: (event: CustomEvent<boolean>) => void;
}) {
  const checkboxRef = useRef<HTMLModusWcCheckboxElement>(null);

  useEffect(() => {
    const checkbox = checkboxRef.current;
    if (!checkbox) return;

    const handleValueChange = (event: Event) => {
      const customEvent = event as CustomEvent<InputEvent>;
      
      // 🚨 CRITICAL BUG WORKAROUND: The ModusWcCheckbox component has a value
      // inversion bug where the `value` property returns the opposite of the
      // actual checked state. This function corrects this by inverting the
      // raw value before passing it to the parent component.
      const rawValue = (customEvent.target as HTMLModusWcCheckboxElement).value;
      const actualValue = !rawValue; // ✅ CORRECT: Invert the value

      // Create a new event with the corrected value
      const correctedEvent = new CustomEvent("valueChange", {
        detail: actualValue,
        bubbles: true,
        cancelable: true,
      });

      onValueChange?.(correctedEvent);
    };

    if (onValueChange) {
      checkbox.addEventListener("inputChange", handleValueChange);
    }

    return () => {
      if (onValueChange) {
        checkbox.removeEventListener("inputChange", handleValueChange);
      }
    };
  }, [onValueChange]);

  return (
    <ModusWcCheckbox
      ref={checkboxRef}
      value={value}
    />
  );
}

Usage in Forms

When using checkboxes in forms:

function MyForm() {
  const [agreeToTerms, setAgreeToTerms] = useState(false);

  return (
    <ModusCheckbox
      value={agreeToTerms}
      onValueChange={(event) => {
        // event.detail is already corrected (inverted)
        setAgreeToTerms(event.detail);
      }}
      label="I agree to the terms and conditions"
    />
  );
}

Multiple Checkboxes

When handling multiple checkboxes:

function CheckboxGroup() {
  const [checkboxes, setCheckboxes] = useState({
    option1: false,
    option2: true,
    option3: false,
  });

  const handleCheckboxChange = (key: string, event: CustomEvent<boolean>) => {
    // event.detail is already corrected
    setCheckboxes(prev => ({
      ...prev,
      [key]: event.detail,
    }));
  };

  return (
    <>
      <ModusCheckbox
        value={checkboxes.option1}
        onValueChange={(e) => handleCheckboxChange('option1', e)}
        label="Option 1"
      />
      <ModusCheckbox
        value={checkboxes.option2}
        onValueChange={(e) => handleCheckboxChange('option2', e)}
        label="Option 2"
      />
      <ModusCheckbox
        value={checkboxes.option3}
        onValueChange={(e) => handleCheckboxChange('option3', e)}
        label="Option 3"
      />
    </>
  );
}

Important Notes

1. The bug is in target.value: The inversion happens when reading event.target.value, not in event.detail
2. Always invert: Never trust the raw value - always invert it
3. Wrapper handles it: The ModusCheckbox wrapper component already handles this, so if you're using the wrapper, you don't need to invert again
4. Document the bug: Add comments explaining the workaround

Testing

To verify the fix works:

// Test that checked state matches expected value
<ModusCheckbox
  value={isChecked}
  onValueChange={(event) => {
    console.log('Raw value (inverted):', event.detail);
    console.log('Expected checked:', isChecked);
    // These should match after inversion
  }}
/>

Common Mistakes

1. Forgetting to invert: Using raw value directly
2. Double inversion: Inverting when using the wrapper component (it already handles it)
3. Wrong event: Using inputChange instead of reading target.value

Related Files

• src/components/ModusCheckbox.tsx - Complete wrapper implementation
• .cursor/rules/modus-checkbox-value-inversion-react.mdc - Detailed documentation