@api recordId property in lighting component will give you record id of current record page .

For example – if your using custom lighting component on account record page then using @api recordId you can get record id of current account record in your LWC .

Now in order to send that recordId to apex class follow me :

Here I am assuming that your having the lightning component and apex class with method ,where your trying to send current recordId from lighting component .

Sample APEX method

@AuraEnabled(cacheable = true)
public static void getCurentRecordId(String recordId){ 
    System.debug('recordid '+ recordId)
}

Now import this method and declare @api recordId into your lighting component .

Here is code of LWC JavaScript file , where we call that getCurentRecordId method into connectedCallback which will send recordId to our apex method.

Sample LWC JavaScript Code

import { LightningElement } from 'lwc';
import {api , track  } from 'lwc'
import getCurentRecordId from '@salesforce/apex/GetRecordId.getCurentRecordId'

export default class GetRecordID extends LightningElement {
    @api recordId;  
    @track currentRecordid = '';
    connectedCallback() { 
        this.currentRecordid = this.recordId;
        curentRecordId(recordId : this.currentRecordid)
        .then((result) => {
            if (result) {
               console.log('Yo yo....found')
            } else {
                console.log('opps something is wrong')
            }
        }).catch((error) => {
            console.log('Catch Error')
        });
    }
}

this how simple it is to send current record page Id your apex method , hope we help you to solve your issue , if you have any question and feel to ask , we are happy to help you.

Thank you…..

Mistakes in the code above

• The JavaScript imports getCurentRecordId but calls curentRecordId(...), a different name.
• The call is written as (recordId : value). It must be an object: { recordId: value }.
• The Apex method is marked cacheable=true but returns void. A cacheable method must return a value.
• The Apex line System.debug(...) has no semicolon.

Corrected Apex

public with sharing class GetRecordId {
    @AuraEnabled(cacheable=true)
    public static String getCurrentRecordId(Id recordId) {
        System.debug('recordId ' + recordId);
        return recordId;
    }
}

Corrected JavaScript

import { LightningElement, api, wire } from 'lwc';
import getCurrentRecordId from '@salesforce/apex/GetRecordId.getCurrentRecordId';

export default class GetRecordId extends LightningElement {
    @api recordId;

    @wire(getCurrentRecordId, { recordId: '$recordId' })
    wiredResult({ data, error }) {
        if (data) {
            console.log('Record Id received in Apex: ', data);
        } else if (error) {
            console.error(error);
        }
    }
}

The $ before recordId makes the wire wait until the page provides the Id. The component receives recordId automatically when it is placed on a record page.

FAQ

How do I get the record Id inside a Visualforce controller?
Use ApexPages.currentPage().getParameters().get('id').

Why is recordId undefined sometimes?
It is set only on record pages and in quick actions. On a home or app page there is no record.