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 (except arrow functions) 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:
-
arguments.callee: the function being invoked -
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
🚨 Optional Chaining has made it to the ES2020/ES11 Specifications! Check it out here: https://tc39.es/ecma262/2020/
Top comments (12)
That thing with
arguments[]array was new to me! Thanks :). dev.to is a really refreshing breeze after StackOverflow experience.I'm glad you discovered something new! Keeps me motivated to write. Thanks for reading ♥️
Note that arrow functions do not have an arguments object.
This is such an oversight from me. Thanks for catching it. I wrote a full blog post on arrow functions and how they behave differently with
this,newandarguments. I missed it here. I'll update.Thanks for reading ♥️
Optional Chaining made it into ES2020, and it's implemented in all major browsers now (caniuse.com/#feat=mdn-javascript_o...)!
Still shows in Stage 4 (Draft).
tc39.es/proposal-optional-chaining/
github.com/tc39/proposal-optional-...
Idk how those pages get updated, but there's no doubt it made it into ES2020: 2ality.com/2019/12/ecmascript-2020...
TC39 is the committee that handles ES proposals and specifications.
That being said your article does lead to what I was eventually looking for
tc39.es/ecma262/2020/
Looks like it is in the final document. I will update the post. Thanks for the awesome feedback ♥️
Yeah I know about TC39, I just meant that I don't know at what point on the process those specific pages get updated; I would have thought it would be after ES2020 is finalized and released, but apparently not
Great post. Slight nit:
new Date().toString() // works
new Date.toString() // fails
I am so glad you brought this up. I would love to explain this! There is a reason why this works:
And this does not
It's because
new Date.toString()is not equal tonew Date().toString(). *There is an extremely subtle difference. They have different precedence. *Check out: developer.mozilla.org/en-US/docs/W...
new Date.toString()throws an error because.has higher precedence thannew Dateso the expression becomes (or it is resolved as)(new (Date.toString))(). They might look same but they are evaluated differently!In short, if you would like to invoke the constructor and chain it with a method in the object, the correct syntax is:
Give it a try😉
That's pretty amazing thanks!
So when chaining you dont save typing the parentheses, they can just go in two different places :)