Александр

function get(url, callback) {
	const request = new XMLHttpRequest();

	request.addEventListener('load', function (event) {
		try {
			const data = JSON.parse(request.response);

			callback(data);
		} catch (error) {
			callback(request.response);
		}
	});

	request.open('GET', url);
	request.send();
}

get('https://jsonplaceholder.typicode.com/todos', function (data) {
	console.log(data);
});

get('URL_1', function (data_1) {
	get('URL_2', function (data_2) {
		get('URL_3', function (data_3) {
			// do something
		});
	});
});

function get(url) {
	return new Promise(function (resolve, reject) {
		const request = new XMLHttpRequest();

		request.addEventListener('load', function (event) {
			try {
				const data = JSON.parse(request.response);

				resolve(data);
			} catch (error) {
				resolve(request.response);
			}
		});

		request.addEventListener('error', function (event) {
			reject({
				status: request.status,
				text: request.statusText
			});
		});

		request.open('GET', url);
		request.send();
	});
}

Promise.all([
	get('URL_1'),
	get('URL_2'),
	get('URL_3')
]).then(function ([data_1, data_2, data_3]) {
	// do something
});

fetch('https://jsonplaceholder.typicode.com/todos')
    .then(response => response.json())
    .then(data => {
        const element = document.querySelector('pre');

        element.textContent = JSON.stringify(data, null, '\t');
    });

for (let i = 0; i < tasks.length; i++) {
    ...
    li.innerHTML = tasks[i];
    ...
}

for (let task of tasks) {
    ...
    li.innerHTML = task;
    ...
}

`some
data`

'some' + 
'data'

class EventEmitter {
	constructor() {
		this.handlers = new Map();
	}

	on(event, handler) {
		const handlers = this.handlers.has(event) ? this.handlers.get(event) : new Set();

		handlers.add(handler);

		this.handlers.set(event, handlers);
	}

	emit(event, ...args) {
		if (this.handlers.has(event)) {
			const handlers = this.handlers.get(event);

			for (let handler of handlers) {
				handler(...args);
			}
		}
	}
}

on(event, callback) {
	this.events.on(event, callback);
}

customSelectInstanse.on('close', () => {
	console.log('Close');
});

totalAmountOfCredit.innerHtml= `${lounAmount} ₽`;
totalMonthlyPayment.innerHtml= `${monthlyPaymentArounded} ₽`;
totalRecommendedIncome.innerHtml= `${monthlyPaymentArounded  + ((monthlyPaymentArounded / 100) * 35)} ₽`

function verifyLogin(login) {
	return new Promise(function (resolve, reject) {
		connection.query(`SELECT login FROM user_data WHERE login = '${login}'`, function (error, results, fields) {
			if (error) {
				reject(error);
			}

			if (results[0] !== undefined) {
				resolve('Логин уже используется');
			} else {
				resolve(0);
			}
		});
	});
}

array.reduce((accumulator, value, index) => {
	if (!(accumulator[index % 2] instanceof Array)) {
		accumulator[index % 2] = [];
	}

	accumulator[index % 2].push(value);

	return accumulator;
}, []);

array.reduce((accumulator, value, index) => {
	accumulator[index % 2].push(value);

	return accumulator;
}, [[], []]);

array.reduce((accumulator, value, index) => (accumulator[index % 2].push(value), accumulator), [[], []]);

array.reduce((a, v, i) => (a[i % 2].push(v), a), [[], []]);

const countOff = (array, count) => array.reduce((accumulator, value, index) => {
	accumulator[index % count].push(value);

	return accumulator;
}, Array.from({ length: count }, () => []));

countOff([1, 2, 3, 4, 5, 6], 2); // [[1, 3, 5], [2, 4, 6]]
countOff([1, 2, 3, 4, 5, 6], 3); // [[1, 4], [2, 5], [3, 6]]

const pow = (number, power) => {
    if (number === 1 || power === 0) {
        return 1;
    }

    const count = Math.round(Math.abs(power));
    let result = number;
    for (let index = 0; index < count - 1; index++) {
        result *= number;
    }

    if (power < 0) {
        return 1 / result;
    } else {
        return result;
    }
};

console.log(Math.pow(2, 2) === pow(2, 2)); // true
console.log(Math.pow(2, -2) === pow(2, -2)); // true
console.log(Math.pow(2, -3) === pow(2, -3)); // true
console.log(Math.pow(3, -4) === pow(3, -4)); // true

function mynumber(a, b) {
	var c = 1;
	if (b < 0) {
		for (var i = b; i < 0; i++) {
			c /= a;
		}
	} else {
		for (var i = 1; i <= b; i++) {
			c *= a;
		}
	}
	return c;
}