Quiz: Do You Know Modern JavaScript?

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Do you know your ES2015 from ES2022?

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Multiple choice. 🤖 … How difficult can it be, eh?

ES2020

What is the value of result?

const a = null;
const b = 100;
const result = a ?? b;
console.log(result);

The null coalescing operator (??) returns the right-hand side operand (b) if the left-hand side (a) is null or undefined. In this case, a is null, so result is 100.

42 null undefined 100

ES2020

What will be the output of the following code?

const value = false;
const defaultVal = 42;
console.log(value ?? defaultVal);

The null coalescing operator (??) treats falsey values like false as valid values. Since value is false, it is considered a valid value and returned.

false 42 null undefined 100

Optional Chaining

What is the output of the following code?

const obj = { foo: null };
const result = obj.foo?.bar;
console.log(result);

The optional chaining operator (?.) stops the evaluation if the left-hand side is null or undefined. Since obj.foo is null, obj.foo?.bar evaluates to undefined.

undefined Error: Cannot read property of undefined null 100

BigInt

What is the output of the following code?

const a = 42n;
const result = a * 2n;
console.log(result);

BigInt values are created by appending n to a number. You cannot mix BigInt with regular numbers in arithmetic operations. Here, both values are BigInt, so the multiplication works, resulting in 84n.

TypeError: Cannot mix BigInt and number 42n 84n undefined

Dynamic Import

What does the import() function return?

const modulePromise = import('./myModule.js');
console.log(typeof modulePromise);

The import() function returns a Promise that resolves to the module object. In this case, typeof modulePromise will return 'object', as it is an instance of Promise.

SyntaxError Promise Module undefined

Promise.allSettled

What will be the result of the following code?

const promises = [
  Promise.resolve('success'),
  Promise.reject('error')
];
Promise.allSettled(promises).then(results => {
  console.log(results[0].status + ': ' + results[0].value);
});

Promise.allSettled returns an array of objects describing the outcome of each promise. The first promise is fulfilled with the value 'success', so the log statement will print Fulfilled: success.

Fulfilled: success Rejected: error Pending null

String.matchAll

What does str.matchAll() return?

const str = 'foo1bar2baz3';
const matches = str.matchAll(/\d/g);
console.log(typeof matches);

String.matchAll returns an iterator of matches, not an array. This iterator can be used to get all the matching groups from a string.

Array of matches Iterator of matches Error: Invalid call null

Import Meta

What does import.meta.url represent?

console.log(import.meta.url);

import.meta is an object that contains metadata about the current module. The import.meta.url property represents the URL of the current module, which can be used to get information about where the script is running.

URL of the current module Current timestamp undefined SyntaxError

Logical Assignment

What is the value of a after the logical assignment?

let a = null;
a ||= 10;
console.log(a);

The logical OR assignment (||=) assigns the right-hand value if the left-hand value is falsy (null, undefined, 0, false, etc.). Since a is null, it is assigned the value 10.

5 10 undefined null

10.

Logical Nullish Assignment

What is the value of b after the nullish assignment?

let b = null;
b ??= 10;
console.log(b);

The nullish coalescing assignment (??=) assigns the right-hand value if the left-hand value is null or undefined. Since b is null, it is assigned the value 10.

5 10 undefined null

11.

WeakRef

What does WeakRef provide?

const obj = { data: 'important' };
const ref = new WeakRef(obj);
console.log(ref.deref());

WeakRef provides a weak reference to an object, which allows the object to be garbage collected if no other references exist. The deref() method returns the original object if it is still available, otherwise undefined.

ReferenceError Weak reference to object null undefined

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