validator.js

const assertValidEmail = (value, errorMessage) => {
    const pattern = /^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;

    if (!pattern.test(value)) {
        throw new Error(errorMessage || translator('error.email.malformatted', { email: value}));
    }
};

const assertAvailableEmail = async (value) => {
    return fetch('/validate_email', {
        // API call details
    }).then(async (response) => {
        if (response.ok) {
            return true;
        }
        
        const data = await response.json();
        
        throw new Error(translator(data.message, data.context));
    });
};

const validate = async (validator, value, errorMessage) => {
    return new Promise(async (resolve, reject) => {
        try {
            await validator(value, errorMessage);
            resolve();
        } catch (err) {
            reject(err);
        }
    });
};

Promise.all([
    validate(assertValidEmail, '[email protected]'),
    validate(assertAvailableEmail, '[email protected]')
]).then(() => {
    // email is valid
}).catch((err) => {
    alert(err.message);
});