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npm install
npx babel-node es2020/bigint // Try other examples tooECMAScript is the scripting language which acts as the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. Each proposal for an ECMAScript feature goes through the following maturity stages:
- Stage 0: Strawman;
- Stage 1: Proposal;
- Stage 2: Draft;
- Stage 3: Candidate;
- Stage 4: Finished.
The variable scoping determines the visibility or accessibility of a variable within the certain part of the program or region. In ES6, both const and let keywords allow developers to declare variables in the block scope.
The let statement declares a block-scoped local variable
The arrow functions provides a more concise syntax for writing function expressions by opting out the function and return keywords using fat arrow(=>) notation. Let's see how this arrow function looks like,
// Function Expression
var multiplyFunc = function(a, b) {
return a * b;
}
console.log(multiplyFunc(2, 5)); // 10
// Arrow function
var multiplyArrowFunc = (a, b) => a * b;
console.log(multiplyArrowFunc(2, 5)); // 10You can also skip parenthesis(()) if the function has exactly one parameter(either zero or more than one parameter). Apart from this, you can wrap braces({}) if the function has more than one expression in the body.
Let's list down all the variations of arrow functions,
//1. Single parameter and single statement
var message = name => console.log("Hello, " + name + "!");
message("Sudheer"); // Hello, Sudheer!
//2. Multiple parameters and single statement
var multiply = (x, y) => x * y;
console.log(multiply(2, 5)); // 10
//3. Single parameter and multiple statements
var even = number => {
if(number % 2) {
console.log("Even");
} else {
console.log("Odd");
}
}
even(5); // odd
//4. Multiple parameters and multiple statements
var divide = (x, y) => {
if(y != 0) {
return x / y;
}
}
console.log(divide(100, 5)); // 20
//5. No parameter and single statement
var greet = () => console.log('Hello World!');
greet(); // Hello World!The classes are introduced as syntactic sugar over existing prototype based inheritance and constructor functions. So this feature doesn't bring new object-oriented inheritance model to JavaScript.
Reflection is the ability of a code to inspect and manipulate variables, properties, and methods of objects at runtime. JavaScript already provides Object.keys(), Object.getOwnPropertyDescriptor(), and Array.isArray() methods as classic refection features. In ES6, it has been officially provided through Reflect object. Reflect is a new global object which is used to call methods, construct objects, get and set properties, manipulate and extend properties. It is similar to Math and JSON objects and all the methods of this object are static.
ES2015/ES6 introduced a huge set of new features. But ECMAScript 2016 Or ES7 introduced only two new features:
- Array.prototype.includes()
- Exponentiation operator
Prior to ES7, you have to use indexOf method and compare the result with '-1' to check whether an array element contains particular element or not.
const array = [1,2,3,4,5,6];
if(array.indexOf(5) > -1 ){
console.log("Found an element");
}Whereas in ES7, array.prototype.includes() method is introduced as a direct approach to determine whether an array includes a certain value among its entries or not.
const array = [1,2,3,4,5,6];
if(array.includes(5)){
console.log("Found an element");
}In addition to this, Array.prototype.includes() handles NaN and Undefined values better than Array.prototype.indexOf() methods. i.e, If the array contains NaN and Undefined values then indexOf() does not return correct index while searching for NaN and Undefined.
let numbers = [1, 2, 3, 4, NaN, ,];
console.log(numbers.indexOf(NaN)); // -1
console.log(numbers.indexOf(undefined)); // -1On the otherhand, includes method is able to find these elements
let numbers = [1, 2, 3, 4, NaN, ,];
console.log(numbers.includes(NaN)); // true
console.log(numbers.includes(undefined)); // trueThe older versions of javascript uses Math.pow function to find the exponentiation of given numbers. ECMAScript 2016 introduced the exponentiation operator, **(similar to other languages such as Python or F#) to calculate the power computation in a clear representation using infix notation.
//Prior ES7
const cube = x => Math.pow(x, 3);
console.log(cube(3)); // 27
//Using ES7
const cube1 = x => x ** 3;
console.log(cube1(3)); // 27In ES6, Promises were introduced to solve the famous callback hell problem. When a series of nested asynchronous functions need to be executed in order, it leads to a callback hell
function task() {
task1((response1) => {
task2(response1, (response2) => {
task3(response2, (response3) => {
// etc...
};
});
});
}But the Chained Promises creates complex flow for asynchronous code.
Async functions were introduced as a combination of promises and generators to give us the possibility of writing asynchronous in a synchronous manner. i.e, This function is going to be declared with the async keyword which enable asynchronous, promise-based behavior to be written in a cleaner style by avoiding promise chains. These functions can contain zero or more await expressions.
Let's take a below async function example,
```js
async function logger() {
let data = await fetch('http://someapi.com/users'); // pause until fetch returns
console.log(data)
}
logger();
```
Similar to Object.keys which iterate over JavaScript object’s keys, Object.values will do the same thing on values. i.e, The Object.values() method is introduced to returns an array of a given object's own enumerable property values in the same order as for...in loop.
const countries = {
IN: 'India',
SG: 'Singapore',
}
Object.values(countries) // ['India', 'Singapore']By the way, non-object argument will be coerced to an object
console.log(Object.values(['India', 'Singapore'])); // ['India', 'Singapore']
console.log(Object.values('India')); // ['I', 'n', 'd', 'i', 'a']The Object.entries() method is introduced to returns an array of a given object's own enumerable string-keyed property [key, value] pairsin the same order as for...in loop.
const countries = {
IN: 'India',
SG: 'Singapore',
}
Object.entries(countries) // [["IN", "India"], ["SG", "Singapore"]]By the way, non-object argument will be coerced to an object
const countries = ['India', 'Singapore'];
console.log(Object.entries(countries)); // [ ['0', 'India'], ['1', 'Singapore']]
const country = 'India';
console.log(Object.entries(country)); // [["0", "I"], ["1", "n"], ["2", "d"], ["3", "i"], ["4", "a"]]
console.log(Object.entries(100)); // [], an empty array for any primitive type because it won't have any own propertiesProperty descriptors describe the attributes of a property. The Object.getOwnPropertyDescriptors() method returns all own property descriptors of a given object.
It provides the below attributes,
- value: The value associated with the property (data descriptors only).
- writable: true if and only if the value associated with the property may be changed
- get: A function which serves as a getter for the property.
- set: A function which serves as a setter for the property.
- configurable: true if and only if the type of this property descriptor may be changed or deleted.
- enumerable: true if and only if this property shows up during enumeration of the property.
The usage of finding property descriptors for any property seems to be as below,
const profile = {
age: 42
};
const descriptors = Object.getOwnPropertyDescriptors(profile);
console.log(descriptors); // {age: {configurable: true, enumerable: true, writable: true }}Some strings and numbers(money, date, timers etc) need to be represented in a particular format. Both padStart() & padEnd() methods introduced to pad a string with another string until the resulting string reaches the supplied length.
- padStart(): Using this method, padding is applied to the left or beginning side of the string.
For example, you may want to show only the last four digits of credit card number for security reasons,
```js
const cardNumber = '01234567891234';
const lastFourDigits = cardNumber.slice(-4);
const maskedCardNumber = lastFourDigits.padStart(cardNumber.length, '*');
console.log(maskedCardNumber); // expected output: "**********1234"
```
- padEnd(): Using this method, padding is applied to the right or ending side of the string.
For example, the profile information padded for label and values as below
const label1 = "Name";
const label2 = "Phone Number";
const value1 = "John"
const value2 = "(222)-333-3456";
console.log((label1 + ': ').padEnd(20, ' ') + value1);
console.log(label2 + ": " + value2); // Name: John
// Phone Number: (222)-333-3456The Atomics is a global object which provides atomic operations to be performed as static methods. They are used with SharedArrayBuffer(fixed-length binary data buffer) objects. The main use cases of these methods are,
-
atomic operations: When memory is shared, multiple threads can read and write the same data in memory. So there would be a chance of loss of data. But atomic operations make sure that predictable values are written and read, that operations are finished before the next operation starts and that operations are not interrupted.
It provides static methods such as add, or, and, xor, load, store, isLockFree etc as demonstrated below.
const sharedMemory = new SharedArrayBuffer(1024); const sharedArray = new Uint8Array(sharedMemory); sharedArray[0] = 10; Atomics.add(sharedArray, 0, 20); console.log(Atomics.load(sharedArray, 0)); // 30 Atomics.sub(sharedArray, 0, 10); console.log(Atomics.load(sharedArray, 0)); // 20 Atomics.and(sharedArray, 0, 5); console.log(Atomics.load(sharedArray, 0)); // 4 Atomics.or(sharedArray, 0, 1); console.log(Atomics.load(sharedArray, 0)); // 5 Atomics.xor(sharedArray, 0, 1); console.log(Atomics.load(sharedArray, 0)); // 4 Atomics.store(sharedArray, 0, 10); // 10 Atomics.compareExchange(sharedArray, 0, 5, 10); console.log(Atomics.load(sharedArray, 0)); // 10 Atomics.exchange(sharedArray, 0, 10); console.log(Atomics.load(sharedArray, 0)); //10 Atomics.isLockFree(1); // true
-
waiting to be notified: Both
wait()andnotify()methods provides ways for waiting until a certain condition becomes true and are typically used as blocking constructs.Let's demonstrate this functionality with reading and writing threads.
First define a shared memory and array
const sharedMemory = new SharedArrayBuffer(1024); const sharedArray = new Int32Array(sharedMemory);
A reading thread is sleeping and waiting on location 0 which is expected to be 10. You can observe a different value after the value overwritten by a writing thread.
Atomics.wait(sharedArray, 0, 10); console.log(sharedArray[0]); // 100
Now a writing thread stores a new value(e.g, 100) and notifies the waiting thread,
Atomics.store(sharedArray, 0, 100); Atomics.notify(sharedArray, 0, 1);
Trailing commas are allowed in parameter definitions and function calls
function func(a,b,) { // declaration
console.log(a, b);
}
func(1,2,); // invocationBut if the function parameter definition or function call only contains a comma, a syntax error will be thrown
function func(,) { // SyntaxError: missing formal parameter
console.log('no args');
};
func(,); // SyntaxError: expected expression, got ','Note: Trailing commas are not allowed in Rest Parameters and JSON.
ES2015 or ES6 introduced both rest parameters and spread operators to convert arguments to array and vice versa using three-dot(...) notation.
-
Rest parameters can be used to convert function arguments to an array
function myfunc(p1, p2, ...p3) { console.log(p1, p2, p3); // 1, 2, [3, 4, 5, 6] } myfunc(1, 2, 3, 4, 5, 6);
-
The spread operator works in the opposite way by converting an array into separate arguments in order to pass to a function
const myArray = [10, 5, 25, -100, 200, -200]; console.log( Math.max(...myArray) ); // 200
ES2018 enables this rest/spread behavior for objects as well.
-
You can pass object to a function
function myfunc({ a, ...x }) { console.log(a, x); // 1, { b: 2, c: 3, d:4 } } myfunc({ a: 1, b: 2, c: 3, d: 4 });
-
The spread operator can be used within other objects
const myObject = { a: 1, b: 2, c: 3, d:4 }; const myNewObject = { ...myObject, e: 5 }; // { a: 1, b: 2, c: 3, d: 4, e: 5 }
Sometimes you may need to avoid duplicate code in the then() and catch() methods.
myPromise
.then(result => {
// process the result and then clean up the resources
})
.catch(error => {
// handle the error and then clean up the resources
});The finally() method is useful if you want to do some processing or resource cleanup once the promise is settled(i.e either fulfilled or rejected).
Let's take a below example to hide the loading spinner after the data is fetched and processed.
let isLoading = true;
fetch('http://somesite.com/users')
.then(data => data.json())
.catch(err => console.error(err))
.finally(() => {
isLoading = false;
console.log('Finished loading!!');
})The flat() method is used to 'flattens' the nested arrays into the top-level array. The functionality of this method is similar to Lodash's _.flattenDepth() function.
const arr = [[1, 2], [[3], 4], [5, 6];
const flattenedArr = arr.flat(2);
console.log(flattenedArr); // => ["1", "2", "3", "4", "5", "6"]In JavaScript, it is very commonn to transforming data from one format. ES2017 introduced Object.entries() method to objects into arrays.
Object to Array:
const obj = {'a': '1', 'b': '2', 'c': '3' };
const arr = Object.entries(obj);
console.log(obj); // [ ['a', '1'], ['b', '2'], ['c', '3'] ]But if you want to get the object back from an array then you need iterate and convert it as below,
js const arr = [ ['a', '1'], ['b', '2'], ['c', '3'] ]; let obj = {} for (let [key, val] of arr) { obj[key] = val; } console.log(obj);
We need a straightforward way to avoid this iteration. In ES2019, Object.fromEntries() method is introduced which performs the reverse of Object.entries() behavior. The above loop can be avoided easily as below,
Iterable( e.g Array or Map) to Object
const arr = [ ['a', '1'], ['b', '2'], ['c', '3'] ];
const obj = Object.fromEntries(arr);
console.log(obj); // { a: "1", b: "2", c: "3" }One of the common case of this method usage is working with query params of an URL,
const paramsString = 'param1=foo¶m2=baz';
const searchParams = new URLSearchParams(paramsString);
Object.fromEntries(searchParams); // => {param1: "foo", param2: "baz"}In order to make consistency with padStart/padEnd, ES2019 provided the standard functions named as trimStart and trimEnd to trim white spaces on the beginning and ending of a string. However for web compatilibity(avoid any breakage) trimLeft and trimRight will be an alias for trimStart and trimEnd respectively.
Let's see the usage with an example,
//Prior ES2019
let messageOne = " Hello World!! ";
console.log(messageOne.trimLeft()); //Hello World!!
console.log(messageOne.trimRight()); // Hello World!!
//With ES2019
let messageTwo = " Hello World!! ";
console.log(messageTwo.trimStart()); //Hello World!!
console.log(messageTwo.trimEnd()); // Hello World!!Prior to ES9, if you don't need error variable and omit the same variable then catch() clause won't be invoked. Also, the linters complain about unused variables. Inorder to avoid this problem, the optional catch binding feature is introduced to make the binding parameter optional in the catch clause. If you want to completely ignore the error or you already know the error but you just want to react to that the this feature is going to be useful.
Let's see the below syntax difference between the versions,
// With binding parameter(<ES9)
try {
···
} catch (error) {
···
}
// Without binding parameter(ES9)
try {
···
} catch {
···
}For example, the feature detection on a browser is one of the most common case
let isTheFeatureImplemented = false;
try {
if(isFeatureSupported()) {
isTheFeatureImplemented = true;
}
} catch (unused) {}In ES6, the classes are introduced to create reusable modules and variables are declared in clousure to make them private. Where as in ES2020, private class variables are introduced to allow the variables used in the class only. By just adding a simple hash symbol in front of our variable or function, you can reserve them entirely for internal to the class.
class User {
#message = "Welcome to ES2020"
login() { console.log(this.#message) }
}
const user = new User()
user.login() // Welcome to ES2020
console.log(user.#message) // Uncaught SyntaxError: Private field '#Note: As shown in the above code, If you still try to access the variable directly from the object then you will receive syntax error.
ES2020 is the current newer version of ECMAScript corresponding to the year 2020. This is the eleventh edition of the ECMAScript Language Specification. Even though this release doesn't bring as many features as ES6, it included some really useful features.
Most of these features already supported by some browsers and try out with babel parser support for unsupported features. This edition is set for final approval by the ECMA general assembly in June, 2020. The ECMAScript 2020 (ES2020) language specification is ready now.
In earlier JavaScript version, there is a limitation of using the Number type. i.e, You cannot safely represent integer values(Number primitive) larger than pow(2, 53). In ES2020,
BigInt is introduced as the 7th primitive type to represent whole numbers(integers with arbitrary precision) larger than pow(2, 53) - 1(or 9007199254740991 or Number.MAX_SAFE_INTEGER). This is been created by appending n to the end of an integer literal or by calling the function BigInt().
// 1. Current number system
const max = Number.MAX_SAFE_INTEGER;
console.log(max + 1) // 9007199254740992
console.log(max + 2) // 9007199254740992
// 2. BigInt representation
const bigInt = 9007199254740991n;
const bigIntConstructorRep = BigInt(9007199254740991); // 9007199254740991n
const bigIntStringRep = BigInt("9007199254740991"); // 9007199254740991n
// 3. Typeof usage
console.log(typeof 1)// number
console.log(typeof 1n)// bigint
console.log(typeof BigInt('1'))// bigint
// 4. Operators
const previousMaxNum = BigInt(Number.MAX_SAFE_INTEGER);
console.log(previousMaxNum + 2n); //9007199254740993n (this was not possible before)
console.log(previousMaxNum -2n); //9007199254740990n
console.log(previousMaxNum * 2n); //18014398509481982n
console.log(previousMaxNum % 2n); //1n
console.log(previousMaxNum / 2n); // 4503599627370495n
// 5. comparison
console.log(1n === 1); // false
console.log(1n === BigInt(1)); // true
console.log(1n == 1); // trueStatic imports supports some of the important use cases such as static analysis, bundling tools, and tree shaking, it is also it's desirable to be able to dynamically load parts of a JavaScript application at runtime.
The new feature dynamic import is introduced to load a module conditionally or on demand. Since it returns a promise for the module namespace object of the requested module, the module can be resolved or import can now be assigned to a variable using async/await as below
<script>
const moduleSpecifier = './message.js';
import(moduleSpecifier)
.then((module) => {
module.default(); // Hello, default export
module.sayGoodBye(); //Bye, named export
})
.catch( err => console.log('loading error'));
</script><script>
(async function() {
const moduleSpecifier = './message.js';
const messageModule = await import(moduleSpecifier);
messageModule.default(); // Hello, default export
messageModule.sayGoodBye(); //Bye, named export
})();
</script>and the imported module appears with both default and named exports
export default () => {
return "Hello, default export";
}
export const sayGoodBye = () => {
return "Bye, named export"
}Note: Dynamic import does not require scripts of type="module"
The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand. This operator replaces || operator to provide default values if you treat empty value or '', 0 and NaN as valid values. This is because the logical OR(||) operator treats(empty value or '', 0 and NaN) as falsy values and returns the right operand value which is wrong in this case. Hence, this operator truely checks for nullish values instead falsy values.
let vehicle = {
car: {
name: "",
speed: 0
}
};
console.log(vehicle.car.name || "Unknown"); // Unknown
console.log(vehicle.car.speed || 90); // 90
console.log(vehicle.car.name ?? "Unknown"); // ""(empty is valid case for name)
console.log(vehicle.car.speed ?? 90); // 0(zero is valid case for speed)In a short note, nullish operator returns a non-nullish value and || operator returns truthy values.
There is String#match method to get all the matches of a string against a regular expression by iterating for each match. However this method gives you the substrings that match.
The String#matchAll() is a new method added to String prototype, which returns an iterator of all results matching a string against a regular expression.
const regex = /t(e)(st(\d?))/g;
const string = 'test1test2';
const matchesIterator = string.matchAll(regex);
Array.from(matchesIterator, result => console.log(result));When you this code in browser console, the matches iterator produces an array for each match including the capturing groups with a few extras.
["test1", "e", "st1", "1", index: 0, input: "test1test2", groups: undefined]
["test2", "e", "st2", "2", index: 5, input: "test1test2", groups: undefined]In JavaScript, Long chains of property accesses is quite error-prone if any of them evaluates to null or undefined value. Also, it is not a good idea to check property existence on each item which in turn leads to a deeply-nested structured if statements.
Optional chaining is a new feature that can make your JavaScript code look cleaner and robust by appending(?.) operator to stop the evaluation and return undefined if the item is undefined or null. By the way, this operator can be used together with nullish coalescing operator to provide default values
let vehicle = {
};
let vehicle1 = {
car: {
name: 'ABC',
speed: 90
}
};
console.log(vehicle.car?.name); // TypeError: Cannot read property 'name' of undefined
console.log(vehicle.car?.name); // Undefined
console.log(vehicle.car?.speed); // Undefined
console.log(vehicle1.car?.name); // ABC
console.log(vehicle1.car?.speed); // 90
console.log(vehicle.car?.name ?? "Unknown"); // Unknown
console.log(vehicle.car?.speed ?? 90); // 90It is really helpful to log(especially to debug errors) about each promise when you are handling multiple promises. The Promise.allSettled() method returns a new promise that resolves after all of the given promises have either fulfilled or rejected, with an array of objects describing the outcome of each promise.
const promise1 = new Promise((resolve, reject) => setTimeout(() => resolve(100), 1000));
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 1000));
Promise.allSettled([promise1, promise2]).then(data => console.log(data)); // [
Object { status: "fulfilled", value: 100},
Object { status: "rejected", reason: undefined}
]As per the output, each outcome object returns status field which denotes either "fulfilled"(value present) or "rejected"(reason present)
Prior to ES2020, you need to write different syntax in different JavaScript environments(cross-platforms) just to access the global object. It is really a hard time for developers because you need to use window, self, or frames on the browser side, global on the nodejs, self on the web workers side.
On the other hand, this keyword can be used inside functions for non-strict mode but it gives undefined in strict mode. If you think about Function('return this')() as a solution for above environments, it will fail for CSP enabled environments(where eval() is disabled).
In the older versions, you can use es6-shim as below,
var getGlobal = function () {
if (typeof self !== 'undefined') { return self; }
if (typeof window !== 'undefined') { return window; }
if (typeof global !== 'undefined') { return global; }
throw new Error('unable to locate global object');
};
var globals = getGlobal();
if (typeof globals.setTimeout !== 'function') {
console.log('no setTimeout in this environment or runtime');
}In ES2020, globalThis property is introduced to provide a standard way of accessing the global this value across environments.
if (typeof globalThis.setTimeout !== 'function') {
console.log('no setTimeout in this environment or runtime');
}The import.meta object was created by the ECMAScript implementation with a null prototype to get context-specific metadata about a JavaScript module.
Let's say you are trying to load my-module from a script,
<script type="module" src="my-module.js"></script>Now you can access meta information(base URL of the module) about the module using the import.meta object
console.log(import.meta); // { url: "file:///home/user/my-module.js" }The above URL can be either URL from which the script was obtained (for external scripts), or the document base URL of the containing document (for inline scripts).
Note: Remember import is not really an object but import.meta is provided as an object which is extensible, and its properties are writable, configurable, and enumerable.
Prior to ES2020, the specifications did not specify in which order for (a in b) should run. Even though most of the javascript engines/browsers loop over the properties of an object in the order in which they were defined, it is not the case with all scenarios. This has been officially standardized in ES2020.
var object = {
'a': 2,
'b': 3,
'c': 4
}
for(let key in object) {
console.log(key); // a b c
}