validate.js

import { LightningElement, track, wire } from 'lwc';
import validateData from '@salesforce/apex/ValidationController.validateData';

export default class MyComponent extends LightningElement {
    @track name;
    @track email;
    @track age;
    @track errors = [];

    handleSubmit() {
        validateData({ name: this.name, email: this.email, age: this.age })
            .then(result => {
                if (result.errors.length > 0) {
                    this.errors = result.errors;
                } else {
                    // proceed with next step in process
                }
            })
            .catch(error => {
                console.error(error);
            });
    }
}