Consider an object from which we want to retrive properties.

const obj = {
  earth: { level: { one: 'soft mud and rocks' } },
  vibrations: [1, 2, { range: 'medium' }],
};

• Use [...].map() for each selector, "vibrations[2].range".replace() to replace square brackets with dots.
• Use "earth.level.one".split('.') to split each selector.
• Use [...].filter() to remove empty values and [...].reduce() to get the value indicated by each selector.

const get = (_obj, selectors) =>
  selectors.map(s =>
  s.replace(/\[([^\[\]]*)\]/g, '.$1.')
    .split('.')
    .filter(t => t !== '')
    .reduce((prev, cur) => prev && prev[cur], _obj)
);

Example

get(obj, 'earth.level.one', 'vibrations[0]', 'vibrations[2].range');
// ['soft mud and rocks', 1, 'medium']

In the browser cookies is store in string format containing key-value pairs.
So, how to parse a browser Cookie string and return an object of all cookie name-value pairs?

• separate each key-value pair using String.prototype.split(';')
• Use Array.prototype.map() and String.prototype.split('=') to separate keys from values in each pair.
• Use Array.prototype.reduce() and decodeURIComponent() to create an object with all key-value pairs.

const parseCookie = str =>
  str
  .split(';')
  .map(v => v.split('='))
  .reduce((acc, v) => {
    acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
    return acc;
  }, {});
  
// cookie in browser: `pkg=math; equation=E%3Dmc%5E2`
parseCookie(document.cookie);
//console: { pkg: 'math', equation: 'E=mc^2' }

for loop in modern JavaScript is rarely talked about although it’s only useful in asynchronous operation scenarios.
But what breaking out early consider the following example:
Matching two array

const smallArray = [0, 2];
const largeArray = Array.from({ length: 1000 }, (_, i) => i);

const areEqual = (a, b) => {
  let result = true;
  a.forEach((x, i) => {
    if (!result) return;
    if (b[i] === undefined || x !== b[i]) result = false;
  });
  return result;
}
areEqual(largeArray, smallArray); // false
// Will loop over all items in `largeArray`

The code isn’t optimized, but it highlights the issue of array methods, such as [].forEach() being unable to break out of the loop early.
To counteract this, we could use a for loop and an early return instead:

...
const areEqual = (a, b) => {
  for (let i in a) {
    if (b[i] === undefined || a[i] !== b[i]) return false;
  }
  return true;
}
areEqual(largeArray, smallArray); // false
// Will only loop until the first mismatch is encountered

The simplest way to create a cookie is to assign a string value to the document.cookie object, for example-

document.cookie = "key1 = value1; key2 = value2; expires = date";
// Example
var now = new Date();
now.setTime(now.getTime() + 1 * 3600 * 1000); //set cookies to expire in 1 hour
document.cookie = `${key} = ${value}; ${opt} = ${opt_value}; expires = ${now.toUTCString()};`;

const cookie = document.cookie will return all cookies in one string much like:
cookie1=value; cookie2=value; cookie3=value;

A callback function is a function that is passed to another function as an argument and is executed after some operation has been completed.
for example of a simple callback function that logs to the console after some operations have been completed.

function extendArray(arr, callback) {
  arr.push(100);
  // then execute the callback function that was passed
  callback();
}

var arr = [1, 2, 3, 4, 5];

extendArray(arr, function() {
  console.log("array has been modified", arr);
});