# Add Component Property > Add a reactive property to an existing Lit web component with proper decorators, types, tests, and documentation - Skill: `igniteui/add-component-property` (Agent Skill) - Install (CLI): `npx skillmds@latest add igniteui/add-component-property` - Raw SKILL.md: https://api.skillmd.com/api/skills/igniteui/add-component-property/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - Author: igniteui (https://skillmd.com/u/igniteui) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/igniteui/add-component-property --- # Add Component Property Adds a reactive property to an existing component, with the documentation, tests and generated metadata that go with it. ## When to Use - "Add an 'orientation' property to the divider component" - "Add a 'variant' property with multiple options" ## Related Skills - [create-new-component](../create-new-component/) - Create a component first - [update-component-styles](../update-component-styles/) - Style changes driven by the property ## Required Context - **Component**: which component to modify - **Name and type**: camelCase property, kebab-case attribute - **Default value**: booleans must default to `false` - **Reflection**: only for primitives that affect styling or accessibility - **Purpose**: the description that ships in the public API docs ## Steps ### 1. Declare the property Place it in the `//#region Public attributes and properties` section, in the shape that matches its type: ```ts /** * The style variant of the component. * @attr variant * @default 'primary' */ @property({ reflect: true }) public variant: StyleVariant = 'primary'; /** * Whether user interaction with the component is disabled. * @attr * @default false */ @property({ type: Boolean, reflect: true }) public disabled = false; /** * The number of items rendered per page. * @attr items-per-page * @default 10 */ @property({ type: Number, attribute: 'items-per-page' }) public itemsPerPage = 10; /** The items rendered by the component. */ @property({ attribute: false }) public items: Array = []; ``` Rules that are easy to get wrong: - **Booleans must default to `false`.** An attribute's presence equates to `true`, so a `true` default cannot be turned off from markup. Rename the property instead (`enabled` → `disabled`). - **Never reflect objects or arrays**, and give them `attribute: false` so Lit doesn't try to serialize them. - Lit derives the attribute name automatically, but spell it out for multi-word properties and for HTML look-alikes (`readOnly` → `readonly`, `minLength` → `minlength`). - A read-only value is a getter, not a `readonly @property`. ### 2. Write the description The JSDoc is copied **verbatim** into `custom-elements.json`, the generated Storybook metadata and the Angular / React / Blazor wrapper docs. - **No `igc-` tag names in prose** — "the select component", not `igc-select`. - **Don't restate that it is an attribute.** `@attr` already says so. - **No `Gets/Sets`.** State what the value is; add a second sentence for side effects. - **Booleans start with "Whether …"** and must describe the `true` state accurately — verify against the implementation, since `hide*`/`disable*` names invert the sentence. - **Present tense**, not "will". ```ts // ❌ Wrong /** * The outlined attribute of the control. * @attr */ // ✅ Right /** * Whether the control has an outlined appearance. * @attr */ ``` Full reference: [create-new-component → Documentation Conventions](../create-new-component/SKILL.md#documentation-conventions). ### 3. React to the change If the property only affects the template, do nothing — just use it in `render()`. If it has side effects or feeds derived state, use the Lit lifecycle hooks, not `@watch`: ```ts // Derived state, before rendering protected override willUpdate(changedProperties: PropertyValues): void { if (changedProperties.has('value')) { this._normalized = this.value.trim(); } } // Side effects that need DOM access protected override update(changedProperties: PropertyValues): void { if (changedProperties.has('disabled')) { this._internals.setARIA({ ariaDisabled: `${this.disabled}` }); } super.update(changedProperties); } ``` Always guard with `changedProperties.has()` and call `super.update()` when overriding `update`. For a form-associated control, a property that participates in constraint validation (`min`, `pattern`, `maxLength`, …) must call `this._validate()` from its setter. ### 4. Add tests ```ts it('is initialized with the proper default value', async () => { const el = await fixture( html`` ); expect(el.propertyName).to.equal(defaultValue); }); it('updates on property change', async () => { const el = await fixture( html`` ); el.propertyName = newValue; await elementUpdated(el); expect(el.propertyName).to.equal(newValue); }); it('reflects to an attribute', async () => { const el = await fixture( html`` ); expect(el.propertyName).to.equal(value); expect(el.getAttribute('property-name')).to.equal(value); }); ``` If the property changes the rendered semantics, extend the a11y audit rather than adding a separate one. ### 5. Regenerate the story metadata The `argTypes`, `args` and the args interface live inside a **generated** `// region default … // endregion` block in `stories/[component-name].stories.ts`. Never edit it by hand: ```bash npm run cem # custom-elements.json from the source JSDoc npm run build:meta # the `// region default` block of each story ``` If the generated description reads badly, fix the JSDoc and regenerate. If the property doesn't appear at all, the story was skipped: the filename must match the tag name (`igc-date-picker` → `date-picker.stories.ts`) and the region fence must be present — a missing fence is a silent no-op. Then wire the property into the story templates, which are hand-written: ```ts export const Basic: Story = { render: (args) => html` Content `, }; ``` ### 6. Verify ```bash npm run check npm run test ``` ## Validation Checklist - [ ] Property declared in the public properties region with the right decorator options - [ ] Booleans default to `false`; complex types use `attribute: false` - [ ] `@attr` and `@default` tags present; description follows the [description rules](#2-write-the-description) - [ ] Lifecycle hook used for side effects, `super.update()` called - [ ] `_validate()` called from setters affecting constraint validation - [ ] Tests cover default, change and reflection - [ ] `npm run cem && npm run build:meta` run; generated story region committed - [ ] Story template uses the new property - [ ] `npm run check` and `npm run test` pass - [ ] CHANGELOG updated if the property is part of a feature or fix ## Common Pitfalls | Symptom | Cause / Fix | | --------------------------------------------- | ---------------------------------------------------------------------- | | Attribute can't be turned off from markup | Boolean defaults to `true` — rename so the default is `false` | | `[object Object]` in the DOM | Complex type without `attribute: false` | | String `'false'` behaves as `true` | Missing `{ type: Boolean }` in the decorator | | Attribute name is `propertyname` | Multi-word property without an explicit `attribute: 'property-name'` | | Story control missing after adding a property | `npm run build:meta` not run, or the story is being skipped silently | | Story description reverts | The generated region was hand-edited — fix the JSDoc instead | ## Reference Examples - `src/components/badge/badge.ts` — reflected string, boolean and union-typed properties - `src/components/input/input.ts` — validation-affecting setters calling `_validate()` - `src/components/combo/combo.ts` — complex, non-attribute properties