Constructor brackets are optional

const newDate = new Date(); // valid
const myClass = new MyClass(); // valid

const anotherDate = new Date; // Also valid
const myClass = new MyClass; // You bet this is valid

The only time you would need those brackets is if a constructor expects arguments.

With statement

🚨with statement is not recommended, and it is forbidden in ES5 strict mode.

with statement extends the scope chain for a statement. with will add up all properties of an object passed in the scope chain.

const person = {
    name: "Parwinder",
    age: 33,
    work: "Software Architect"
}

with (person) {
    console.log(`Hi, I am ${name}, and I am ${ age } years old. I work as a ${work}.`);
    // Hi, I am Parwinder, and I am 33 years old. I work as a Software Architect.
}

Function arguments

Every function has an arguments array-like object that contains the value of all arguments passed to the function.

function foo(a, b, c) {
  console.log(arguments[0]); // 1
  console.log(arguments[1]); // 2
  console.log(arguments[2]); // 3
}

foo(1, 2, 3);

arguments have two properties:

  1. arguments.callee: the function being invoked
  2. arguments.callee.caller: the function that has invoked the current function

🚨Just like the with statement above, callee and caller are forbidden in ES5 strict mode.

Pure object

A pure object has no functions in its prototype.

const x = {};

This creates an object, but the prototype will have a constructor and methods like hasOwnProperty, isPrototypeOf and toString.

const x = Object.create(null);

create(null) generates an object with no prototype! 🤯

Removing duplicates from an array

const arr = [1, 1, 1, 1, 2, 3, 4, 5, 6, 7, 6, 6, 6, 7, 8, 9];
const arrWithoutDuplicates = [...new Set(arr)];
console.log(arrWithoutDuplicates); // [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

The key property of a set is having unique values. Once we have the Set from an array, we can use the spread(…) operator to spread it into an empty array.

Optional Chaining

Whenever you access a nested object property where you do not know if the sub object exists or not, you end up doing this:

const nestedObject = {
    name: "Parwinder",
    details: {
        age: 33,
        cars: {
            first: "jeep",
            second: "tesla",
            accessories: {
                x: 200,
                y: 300
            }
        }
    }
}

if (nestedObject &&
    nestedObject.details &&
    nestedObject.details.cars &&
    nestedObject.details.cars.accessories) {
    console.log(nestedObject.details.cars.accessories.x); // 200
}

Optional chaining eliminates the clutter. With optional chaining you can do:

const nestedObject = {
    name: "Parwinder",
    details: {
        age: 33,
        cars: {
            first: "jeep",
            second: "tesla",
            accessories: {
                x: 200,
                y: 300
            }
        }
    }
}

console.log(nestedObject?.details?.cars?.accessories?.x); // 200

🚨At the time of writing, optional chaining is in Stage 4 (Draft) of the new ES standard, and it will most likely make it to the final spec. 🤞