ekrem-aktas

async function validateUsername(username) {
    if (typeof username !== "string" || username.trim() === "") {
        return { valid: false, reason: "Username is needed" };
    }

    if (!username.match(/^\w+$/)) {
        return { valid: false, reason: "Username must consist of characters and numbers" };
    }

    const response = await fetch(`/api/checkusername/${username}`); // returns { available: true | false }

ekrem-aktas

async function isUsernameAvailable(username) {
    const response = await fetch(`/api/checkusername/${username}`); // returns { available: true | false }
    return (await response.json()).available;
}

async function validateUsername(username, usernameUniquenessCheckFn) {
    if (typeof username !== "string" || username.trim() === "") {
        return { valid: false, reason: "Username is needed" };
    }

ekrem-aktas

// Returns a function which runs given checkFn against a value.
const createRule = (checkFn, errorMessage) => {
    return async (value) => await Promise.resolve(checkFn(value)) ? null : errorMessage;
}

// Returns a function which accepts a value to validate and runs all passed rules.
const defineValidator = (...rules) => {
    return async (value) => {    		
        for (const rule of rules) {        		
            const errorMessage = await rule(value);