awesome-copilot/instructions/lwc.instructions.md

description, applyTo

description	applyTo
Guidelines and best practices for developing Lightning Web Components (LWC) on Salesforce Platform.	force-app/main/default/lwc/**

LWC Development

General Instructions

• Each LWC should reside in its own folder under force-app/main/default/lwc/.
• The folder name should match the component name (e.g., myComponent folder for the myComponent component).
• Each component folder should contain the following files:
  • myComponent.html: The HTML template file.
  • myComponent.js: The JavaScript controller file.
  • myComponent.js-meta.xml: The metadata configuration file.
  • Optional: myComponent.css for component-specific styles.
  • Optional: myComponent.test.js for Jest unit tests.

Core Principles

1. Use Lightning Components Over HTML Tags

Always prefer Lightning Web Component library components over plain HTML elements for consistency, accessibility, and future-proofing.

Recommended Approach

<!-- Use Lightning components -->
<lightning-button label="Save" variant="brand" onclick={handleSave}></lightning-button>
<lightning-input type="text" label="Name" value={name} onchange={handleNameChange}></lightning-input>
<lightning-combobox label="Type" options={typeOptions} value={selectedType}></lightning-combobox>
<lightning-radio-group name="duration" label="Duration" options={durationOptions} value={duration} type="radio"></lightning-radio-group>

Avoid Plain HTML

<!-- Avoid these -->
<button onclick={handleSave}>Save</button>
<input type="text" onchange={handleNameChange} />
<select onchange={handleTypeChange}>
    <option value="option1">Option 1</option>
</select>

2. Lightning Component Mapping Guide

HTML Element	Lightning Component	Key Attributes
<button>	<lightning-button>	variant, label, icon-name
<input>	<lightning-input>	type, label, variant
<select>	<lightning-combobox>	options, value, placeholder
<textarea>	<lightning-textarea>	label, max-length
<input type="checkbox">	<lightning-input type="checkbox">	checked, label
<input type="radio">	<lightning-radio-group>	options, type, name
<input type="toggle">	<lightning-input type="toggle">	checked, variant
Custom pills	<lightning-pill>	label, name, onremove
Icons	<lightning-icon>	icon-name, size, variant

3. Lightning Design System Compliance

Use SLDS Utility Classes

Always use Salesforce Lightning Design System utility classes with the slds-var- prefix for modern implementations:

<!-- Spacing -->
<div class="slds-var-m-around_medium slds-var-p-top_large">
    <div class="slds-var-m-bottom_small">Content</div>
</div>

<!-- Layout -->
<div class="slds-grid slds-wrap slds-gutters_small">
    <div class="slds-col slds-size_1-of-2 slds-medium-size_1-of-3">
        <!-- Content -->
    </div>
</div>

<!-- Typography -->
<h2 class="slds-text-heading_medium slds-var-m-bottom_small">Section Title</h2>
<p class="slds-text-body_regular">Description text</p>

SLDS Component Patterns

<!-- Card Layout -->
<article class="slds-card slds-var-m-around_medium">
    <header class="slds-card__header">
        <h2 class="slds-text-heading_small">Card Title</h2>
    </header>
    <div class="slds-card__body slds-card__body_inner">
        <!-- Card content -->
    </div>
    <footer class="slds-card__footer">
        <!-- Card actions -->
    </footer>
</article>

<!-- Form Layout -->
<div class="slds-form slds-form_stacked">
    <div class="slds-form-element">
        <lightning-input label="Field Label" value={fieldValue}></lightning-input>
    </div>
</div>

4. Avoid Custom CSS

Use SLDS Classes

<!-- Color and theming -->
<div class="slds-theme_success slds-text-color_inverse slds-var-p-around_small">
    Success message
</div>

<div class="slds-theme_error slds-text-color_inverse slds-var-p-around_small">
    Error message
</div>

<div class="slds-theme_warning slds-text-color_inverse slds-var-p-around_small">
    Warning message
</div>

Avoid Custom CSS (Anti-Pattern)

/* Don't create custom styles that override SLDS */
.custom-button {
    background-color: red;
    padding: 10px;
}

.my-special-layout {
    display: flex;
    justify-content: center;
}

When Custom CSS is Necessary

If you must use custom CSS, follow these guidelines:

• Use CSS custom properties (design tokens) when possible
• Prefix custom classes to avoid conflicts
• Never override SLDS base classes

/* Custom CSS example */
.my-component-special {
    border-radius: var(--lwc-borderRadiusMedium);
    box-shadow: var(--lwc-shadowButton);
}

5. Component Architecture Best Practices

Reactive Properties

import { LightningElement, track, api } from 'lwc';

export default class MyComponent extends LightningElement {
    // Use @api for public properties
    @api recordId;
    @api title;

    // Primitive properties (string, number, boolean) are automatically reactive
    // No decorator needed - reassignment triggers re-render
    simpleValue = 'initial';
    count = 0;

    // Computed properties
    get displayName() {
        return this.name ? `Hello, ${this.name}` : 'Hello, Guest';
    }

    // @track is NOT needed for simple property reassignment
    // This will trigger reactivity automatically:
    handleUpdate() {
        this.simpleValue = 'updated'; // Reactive without @track
        this.count++; // Reactive without @track
    }

    // @track IS needed when mutating nested properties without reassignment
    @track complexData = {
        user: {
            name: 'John',
            preferences: {
                theme: 'dark'
            }
        }
    };

    handleDeepUpdate() {
        // Requires @track because we're mutating a nested property
        this.complexData.user.preferences.theme = 'light';
    }

    // BETTER: Avoid @track by using immutable patterns
    regularData = {
        user: {
            name: 'John',
            preferences: {
                theme: 'dark'
            }
        }
    };

    handleImmutableUpdate() {
      // No @track needed - we're creating a new object reference
      this.regularData = {
        ...this.regularData,
        user: {
          ...this.regularData.user,
          preferences: {
            ...this.regularData.user.preferences,
            theme: 'light'
          }
        }
      };
    }

    // Arrays: @track is needed only for mutating methods
    @track items = ['a', 'b', 'c'];

    handleArrayMutation() {
      // Requires @track
      this.items.push('d');
      this.items[0] = 'z';
    }

    // BETTER: Use immutable array operations
    regularItems = ['a', 'b', 'c'];

    handleImmutableArray() {
      // No @track needed
      this.regularItems = [...this.regularItems, 'd'];
      this.regularItems = this.regularItems.map((item, idx) =>
        idx === 0 ? 'z' : item
      );
    }

    // Use @track only for complex objects/arrays when you mutate nested properties.
    // For example, updating complexObject.details.status without reassigning complexObject.
    @track complexObject = {
      details: {
        status: 'new'
      }
    };
}

Event Handling Patterns

// Custom event dispatch
handleSave() {
    const saveEvent = new CustomEvent('save', {
        detail: {
            recordData: this.recordData,
            timestamp: new Date()
        }
    });
    this.dispatchEvent(saveEvent);
}

// Lightning component event handling
handleInputChange(event) {
    const fieldName = event.target.name;
    const fieldValue = event.target.value;

    // For lightning-input, lightning-combobox, etc.
    this[fieldName] = fieldValue;
}

handleRadioChange(event) {
    // For lightning-radio-group
    this.selectedValue = event.detail.value;
}

handleToggleChange(event) {
    // For lightning-input type="toggle"
    this.isToggled = event.detail.checked;
}

6. Data Handling and Wire Services

Use @wire for Data Access

import { getRecord } from 'lightning/uiRecordApi';
import { getObjectInfo } from 'lightning/uiObjectInfoApi';

const FIELDS = ['Account.Name', 'Account.Industry', 'Account.AnnualRevenue'];

export default class MyComponent extends LightningElement {
    @api recordId;

    @wire(getRecord, { recordId: '$recordId', fields: FIELDS })
    record;

    @wire(getObjectInfo, { objectApiName: 'Account' })
    objectInfo;

    get recordData() {
        return this.record.data ? this.record.data.fields : {};
    }
}

7. Error Handling and User Experience

Implement Proper Error Boundaries

import { ShowToastEvent } from 'lightning/platformShowToastEvent';

export default class MyComponent extends LightningElement {
    isLoading = false;
    error = null;

    async handleAsyncOperation() {
        this.isLoading = true;
        this.error = null;

        try {
            const result = await this.performOperation();
            this.showSuccessToast();
        } catch (error) {
            this.error = error;
            this.showErrorToast(error.body?.message || 'An error occurred');
        } finally {
            this.isLoading = false;
        }
    }

    performOperation() {
      // Developer-defined async operation
    }

    showSuccessToast() {
        const event = new ShowToastEvent({
            title: 'Success',
            message: 'Operation completed successfully',
            variant: 'success'
        });
        this.dispatchEvent(event);
    }

    showErrorToast(message) {
        const event = new ShowToastEvent({
            title: 'Error',
            message: message,
            variant: 'error',
            mode: 'sticky'
        });
        this.dispatchEvent(event);
    }
}

8. Performance Optimization

Conditional Rendering

Prefer lwc:if, lwc:elseif and lwc:else for conditional rendering (API v58.0+). Legacy if:true / if:false are still supported but should be avoided in new components.

<!-- Use template directives for conditional rendering -->
<template lwc:if={isLoading}>
    <lightning-spinner alternative-text="Loading..."></lightning-spinner>
</template>
<template lwc:elseif={error}>
    <div class="slds-theme_error slds-text-color_inverse slds-var-p-around_small">
        {error.message}
    </div>
</template>
<template lwc:else>
    <template for:each={items} for:item="item">
        <div key={item.id} class="slds-var-m-bottom_small">
            {item.name}
        </div>
    </template>
</template>

<!-- Legacy approach (avoid in new components) -->
<template if:true={isLoading}>
    <lightning-spinner alternative-text="Loading..."></lightning-spinner>
</template>
<template if:true={error}>
    <div class="slds-theme_error slds-text-color_inverse slds-var-p-around_small">
        {error.message}
    </div>
</template>
<template if:false={isLoading}>
  <template if:false={error}>
    <template for:each={items} for:item="item">
        <div key={item.id} class="slds-var-m-bottom_small">
            {item.name}
        </div>
    </template>
  </template>
</template>

9. Accessibility Best Practices

Use Proper ARIA Labels and Semantic HTML

<!-- Use semantic structure -->
<section aria-label="Product Selection">
    <h2 class="slds-text-heading_medium">Products</h2>

    <lightning-input
        type="search"
        label="Search Products"
        placeholder="Enter product name..."
        aria-describedby="search-help">
    </lightning-input>

    <div id="search-help" class="slds-assistive-text">
        Type to filter the product list
    </div>
</section>

Common Anti-Patterns to Avoid

• Direct DOM Manipulation: Never use document.querySelector() or similar
• jQuery or External Libraries: Avoid non-Lightning compatible libraries
• Inline Styles: Use SLDS classes instead of style attributes
• Global CSS: All styles should be scoped to the component
• Hardcoded Values: Use custom labels, custom metadata, or constants
• Imperative API Calls: Prefer @wire over imperative import calls when possible
• Memory Leaks: Always clean up event listeners in disconnectedCallback()