Imperative vs Declarative Apex Method Calls in LWC

Understand the key differences between Imperative and Declarative Apex method calls in LWC — and know exactly when to use each approach.

1. Introduction

Apex communication is essential for Lightning Web Components (LWC) because it bridges the gap between the client-side (your component's JavaScript) and the server-side (your Salesforce org's data and logic).

In LWC, there are two ways to call server-side Apex methods:
• Imperative — Manually call the method when needed (e.g., on button click).
• Declarative — Automatically fetch data when the component loads using @wire.

This is a crucial concept for efficient and performant data handling in your LWC components.

2. Imperative (Manual Call)

Calls the Apex method on demand (e.g., when a button is clicked). Fetch data only when needed (e.g., user-triggered actions).

import { LightningElement } from 'lwc';
import getAccount from '@salesforce/apex/AccountController.getAccount';

export default class ImperativeExample extends LightningElement {
    accountId = '001xx000003DGb0';

    // Imperative call (triggered by a button click)
    handleClick() {
        getAccount({ accountId: this.accountId })
            .then(data => {
                console.log('Account Data:', data);
            })
            .catch(error => {
                console.error('Error:', error);
            });
    }
}

<template>
    <button onclick={handleClick}>Load Account</button>
</template>

• The Apex method is called only when the button is clicked — not on component load.
• Uses .then() to handle the successful result and .catch() to handle errors.
• Gives you full control over when and how the data is fetched.

3. Declarative (Using @wire)

Automatically fetches data when the component loads (like a subscription). Fetch data immediately when the component renders (e.g., load a record on page load).

import { LightningElement, wire } from 'lwc';
import getAccount from '@salesforce/apex/AccountController.getAccount';

export default class DeclarativeExample extends LightningElement {
    // Declarative call using @wire
    @wire(getAccount, { accountId: '001xx000003DGb0' })
    wiredAccount({ data, error }) {
        if (data) {
            console.log('Account Data:', data);
        } else if (error) {
            console.error('Error:', error);
        }
    }
}

How it works: The @wire decorator automatically calls the Apex method getAccount when the component loads. If the accountId changes, the method re-runs automatically.

4. Key Differences

Aspect	Imperative	Declarative
Timing	Manually Triggered (e.g., button click)	Automatically on component load
Data Reactivity	Requires Manual Refresh	Updates if parameters change
Use Case	User-Triggered Actions	Initial data loading
Error Handling	Uses .then() and .catch()	Uses { data, error } in the wire method

5. When to Use Which?

Use Declarative (@wire) when:
• You need a straightforward way to load data when the component is initialized.
• You want to leverage automatic updates to your component when the data changes.
• Lightning Data Service (LDS) handles caching and sharing data between components, which can boost performance.

Use Imperative when:
• The data retrieval depends on user interactions (e.g., button clicks, form submissions).
• You need to handle complex business logic before calling the Apex method.
• You need to perform DML operations (insert, update, delete) — @wire only supports read-only operations.
• When caching is not beneficial or you need fresh data every time.

6. Quick Reference

Imperative syntax:

// Call on button click or any event
apexMethodName({ param1: value1 })
    .then(result => { /* handle result */ })
    .catch(error => { /* handle error */ });

Declarative syntax (wire as property):

@wire(apexMethodName, { param1: value1 })
propertyName;

Declarative syntax (wire as function):

@wire(apexMethodName, { param1: '$reactiveParam' })
wiredMethodName({ data, error }) {
    if (data) { /* handle data */ }
    else if (error) { /* handle error */ }
}

📝 Remember: Use cacheable=true on your Apex method annotation when using @wire. For imperative calls that perform DML, do NOT use cacheable=true.