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);            

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

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 validateUsername(username) {
    const errorNode = document.getElementById("error-text");

    // Check if username is a non-empty string
    if (typeof username !== "string" || username.trim() === "") {
        errorNode.innerText = "Username is required";
        return false;
    }

    // Check if username format is valid

ekrem-aktas

function groupBy(arr, func){
  let grouped = Object.create(null);
  
  arr.forEach((item) => {
    const key = func(item);   
    if (!grouped[key]) {
      grouped[key] = [];
    }
    grouped[key].push(item);
  });

ekrem-aktas

function groupBy(arr, func){
  return arr
    .reduce((acc, item) => {
       const key = func(item);
       if (!acc[key]) {
         acc[key] = [];
       }
       acc[key].push(item);
       return acc;
    }, Object.create(null));

ekrem-aktas

const members = [["name", "apple"], ["type", "fruit"]];

const obj = Object.create(null);
members.forEach(([key, value]) => obj[key] = value);

console.log(obj); // { name: "apple", type: "fruit" }

ekrem-aktas

const members = [["name", "apple"], ["type", "fruit"]];

const food = members.reduce((obj, [key, value]) => {
  obj[key] = value;
  return obj;
}, Object.create(null));

console.log(food); // { name: "apple", type: "fruit" }

ekrem-aktas

const numbers = [1, 2, 3];

let total = 0;
numbers.forEach(num => total += num);

console.log(total) // 6

ekrem-aktas

const numbers = [1, 2, 3];
const total = numbers.reduce((sum, num) => sum + num);

console.log(total); // 6