LWC Constraints

When to Use

This skill auto-activates when writing, reviewing, or modifying any Lightning Web Component, template, or JavaScript controller. It enforces naming, security, accessibility, and performance rules for all LWC artifacts.

Hard rules for Lightning Web Component development. Violations must be flagged or fixed before code is considered complete.

For lifecycle hooks, reactive property rules, wire service rules, event propagation, and slot rules see @../_reference/LWC_PATTERNS.md.

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Naming Conventions

Element	Convention	Example
Component folder & files	camelCase	accountList/accountList.js
HTML tag in markup	kebab-case with namespace	<c-account-list>
Public properties (@api)	camelCase in JS, kebab-case in HTML	JS: @api maxRecords / HTML: max-records="10"
Private fields	camelCase, prefix _ for backing fields	_wiredResult, isLoading
Custom events	lowercase, no spaces, hyphens allowed	opportunityselect, row-action
CSS classes	SLDS utilities or BEM for custom	slds-p-around_medium, card__title

Never use PascalCase or UPPER_CASE for component folder names or file names.

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Never Do

Rule	Reason
Direct DOM manipulation (document.createElement, innerHTML, appendChild)	Breaks Shadow DOM encapsulation and LWS security model. Use template directives (lwc:if, for:each) and reactive state instead.
Imperative Apex when @wire works	Wire provides caching via LDS, automatic re-provisioning on param change, and offline support. Use imperative calls only for DML or non-cacheable operations.
Inline styles (style="color:red")	Violates SLDS theming, breaks dark mode (SLDS 2.0), and bypasses component-scoped CSS. Use SLDS utility classes or CSS custom properties.
querySelector('#id') for shadow or slotted elements	Element IDs are transformed at render time. Use this.template.querySelector('.class') for shadow elements, this.querySelector('.class') for light DOM.
Update @wire config inside renderedCallback()	Creates an infinite re-render loop: render -> renderedCallback -> wire config change -> re-provision -> re-render.
Mutate @wire result data directly	Wire data is immutable (read-only). Shallow-copy ({...data} or [...data]) before mutating.
Use deprecated if:true / if:false directives	Replaced by lwc:if / lwc:elseif / lwc:else (GA). Deprecated directives will be removed.
Skip super() in constructor()	Required by the Custom Elements spec. Omitting it throws a runtime error.
Access DOM in constructor()	Shadow DOM is not attached yet. No this.template, no child elements, no attributes.
Dispatch events with bubbles:true, composed:true without namespacing	Composed events cross all shadow boundaries and become public API. Namespace the event name to avoid collisions.
Use localStorage/sessionStorage without namespace awareness	Under LWS, storage is namespace-scoped. Keys that worked under Locker may behave differently.
Heavy logic in renderedCallback() without a guard	Fires on every render cycle. Unguarded work causes performance degradation or infinite loops if it updates reactive state.

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Always Do

Rule	Detail
Use @api for all public properties and methods	Defines the component's contract. Without @api, properties are private and invisible to parent components and App Builder.
Use kebab-case for component tags in HTML	<c-account-list>, never <c-accountList>. The framework maps kebab-case tags to camelCase folder names automatically.
Use camelCase for JS properties	@api maxRecords, not @api max_records. Kebab-to-camelCase mapping applies when attributes are set in HTML.
Provide alternative-text on <lightning-spinner>	Required for screen-reader accessibility. Omitting it is a WCAG violation.
Provide label on all <lightning-input> elements	Required for accessibility. Use variant="label-hidden" if visual label must be hidden, but never omit label entirely.
Set key on iterated elements	for:each requires a unique, stable key attribute (use record Id, not array index). Missing keys cause rendering bugs.
Use lwc:if / lwc:elseif / lwc:else for conditionals	Modern directive (GA). Always prefer over deprecated if:true/if:false.
Implement errorCallback(error, stack) or wrap with error boundary	Catches descendant lifecycle and render errors. Without it, a child error crashes the entire component tree.
Clean up in disconnectedCallback()	Remove window/document event listeners, unsubscribe() from message channels, clear timers. Prevents memory leaks.
Store bound references for event listeners	this._boundHandler = this.handler.bind(this) in constructor, so removeEventListener gets the same reference as addEventListener.
Set <apiVersion>66.0</apiVersion> in .js-meta.xml	Target Spring '26. Older API versions miss LWS, SLDS 2.0 styling hooks, and new wire adapters.
Use WITH USER_MODE in backing Apex SOQL	Enforces FLS/CRUD. Required for security review. The LWC itself does not bypass object permissions, but its Apex must not either.
Prefer SLDS utility classes over custom CSS	Ensures dark mode compatibility (SLDS 2.0), consistent spacing, and responsive layout.

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Anti-Pattern Table

Anti-Pattern	Correct Pattern	Why
document.querySelector('.my-class')	this.template.querySelector('.my-class')	Shadow DOM scoping. document queries cannot reach inside shadow roots.
@track on primitives or simple reassignments	Remove @track; all fields are reactive since Spring '20	@track is only needed for deep observation of object/array mutations in place. Overuse adds confusion.
Imperative Apex in connectedCallback for cacheable reads	@wire(getRecord, ...) or @wire(apexMethod, ...)	Wire caches via LDS, re-provisions automatically, and works offline. Imperative calls bypass the cache.
setTimeout to wait for DOM	renderedCallback() with a guard flag	The DOM is guaranteed available in renderedCallback. setTimeout is fragile and race-prone.
event.target inside shadow boundary listeners	event.currentTarget or event.target.dataset.*	event.target is retargeted at shadow boundaries. Use currentTarget for the element the listener is attached to.
Hardcoded field API names as strings	Import from @salesforce/schema	Schema imports give compile-time validation, refactoring safety, and dependency tracking.
console.log left in production code	Remove or guard with IS_DEBUG flag	Console output is visible to end users in browser dev tools and degrades performance at scale.
Fetching all records then paginating client-side	Server-side pagination via OFFSET/LIMIT or SOQL cursors (v66.0+)	Client-side pagination loads all data upfront; breaks on large datasets.
if:true={property}	lwc:if={property}	if:true/if:false are deprecated. lwc:if supports elseif/else chains and will be the only option going forward.
Inline style="width:100px"	CSS class or SLDS utility slds-size_full	Inline styles bypass theming, break dark mode, and cannot be overridden by styling hooks.
Missing .js-meta.xml <targets>	Declare all intended surfaces (RecordPage, AppPage, etc.)	Component will not appear in App Builder or Flow without explicit target declarations.

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Wire Service Constraints

1. All $-prefixed reactive params must be defined (non-undefined) before the wire adapter fires.
2. Wire data arrives non-deterministically -- never assume it is available in connectedCallback.
3. Wire chains ($record.data.fieldName) are valid but add latency; keep chains to two hops max.
4. Use refreshApex(wiredResult) after imperative DML -- store the full wire result reference (_wiredResult) for this purpose.
5. For @wire with Apex, the Apex method must be annotated @AuraEnabled(cacheable=true).

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Event Constraints

1. Default to { bubbles: false, composed: false } for parent-child communication.
2. Always use CustomEvent with a detail property for data payload -- never custom properties on the event object.
3. Parent listens with on{eventname} (all lowercase, no hyphens in the on prefix).
4. If composed: true is required, namespace the event name (e.g., myns__recordupdate) to prevent collisions.

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Accessibility Constraints

1. Every interactive element must be keyboard-operable (focusable, activatable via Enter/Space).
2. Every <lightning-spinner> must have alternative-text.
3. Every <lightning-input> must have label (use variant="label-hidden" to visually hide).
4. Every <lightning-icon> used as a meaningful indicator must have alternative-text; decorative icons should set aria-hidden="true".
5. Color must never be the sole indicator of state -- pair with text, icon, or ARIA attributes.
6. Custom components exposing interactive regions should set appropriate role and aria-* attributes.

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Meta XML Constraints

1. Always include <apiVersion>66.0</apiVersion> (Spring '26).
2. Set <isExposed>true</isExposed> for any component intended for App Builder, Flow, or Experience Cloud.
3. Declare every intended surface in <targets>.
4. Expose configurable @api properties via <targetConfigs> with label, type, and description.