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Bridging Legacy code to ES-Next

Intro#

ES6 - The Future Is Now! (Sort of)#

Lesson Objectives#

  • Know what ESNext is
  • Know how to read MDN for compatibility
  • Explain the purpose of Babel/Transpiling/Compiling
  • Exposure to many new methods/syntaxes of ES6 in order to get ready for React

Introduction#

JavaScript was created in the 1990s by Brendan Eich in just 10 days. It's main function was to work in web browsers, mainly for manipulating the DOM. It was originally called Mocha, then LiveScript and then finally renamed to JavaScript - this was mostly done for marketing purposes.

Though Brendan Eich is a brilliant man, 10 days is not enough to create a great language. JavaScript was problematic and it also struggled in that it had to run in all the different browsers, so making updates that would work across all browsers was really really tough - especially, back when a CD (disk) was required to update programs.

As technology progressed, releasing new updates to browsers became faster and easier and the web moved towards more small updates rather than massive overhauls.

This changed allowed JavaScript to advance as well. JavaScript specifications and standards are maintained by ECMA (European Computer Manufacturers Association) on an international level, hence the alternate name ES or ECMAScript. Reference

On the job, you'll be exposed to legacy code, current standards and sometimes bleeding edge technology.

Today we're moving forward with some of the latest features of ESNext that are quickly becoming standard.

It is also time to move past fundamentals and start building your skills of learning on your own. The goal of this lesson is to expose you to MANY (definitely TOO MANY) new features of ESNext. The goal isn't to learn them all today; rather, the goal is to raise your awareness so when you start seeing some of this code you'll know you can look it up and learn about it more. Most of the selected examples are required for learning React, but as always, give yourself time to practice and build your understanding.

Topics#

  • What is ESNext
  • strict mode (legacy)
  • understanding MDN
  • polyfills and shims
  • IFFE
  • Block level scoping with let
  • default value setting
  • arrow functions
  • spread and rest operators
  • swap (desconstucting method)
  • Object literal extensions

ES6#

ECMAScript is the standard version of JavaScript. ECMA stands for European Computer Manufacturers Association. ES5 became standard in 2009. ES6 (or ES2015), became standard in 2015. But just because it is made standard, it doesn't mean that all browsers/environments are ready/compliant.

ES6 was the first major overhaul of JavaScript in 6 years (the previous major updates were in 2009 and 1999). Since the release of release, there have been much smaller updates annually. Each upcoming release is referred to as ESNext.

Strict Mode/Versioning#

Software releases usually come with a set of 3 numbers and many (but not all) follow the following pattern

ie: 12.4.3

The first number is a major change that usually involves breaking changes - previous code that has been written that works just fine can totally break/be incompatible with some changes.

The second number is a minor change, often involving something like a new feature and typically doesn't break already working code.

The third usually denotes bug fixes and security updates.

Because ES6 was going to have breaking changes, one thing that was as built-in was to add the text use strict at the top of a file. If someone wrote code ES5 code that would break with the changes coming with ES6 the code would error. This would help ensure that programs that would be transitioning from ES5 to ES6 would not break.

In the year 2019(nearly 2020), use strict is no longer needed. But you may see it in older code.

More info

MDN#

Lets you know which methods/syntaxes are standard, experimental, depreciated, when things were added and more including shims/polyfills. forEach on MDN

Quickly see if a method is standard[no notation], experimental[flask], depreciated [removed from standards, may be removed](thumbs down), or obsolete (likely removed altogether in newer environments) [trash can]

Screenshot left bar, objects

See more information about when the method was defined implemented: Screenshot- forEach Specifications section

Polyfills/Shims#

Some people call them polyfills, some call them shims; either way: It is code that is a fallback that provides the functionality when the expected functionality is not available (because old browser, etc. ). Many entries in MDN have polyfills. This is also a great way to look at how the ECMAScript standard code is written ( a lot of examples are directly from the official ECMAScript docs)

An example Polyfill Screenshot- forEach polfill section

When can I use these new standards?#

Now! But it depends where. Node.js support most/nearly all new things. Chrome and Safari, at this moment in time, have been keeping up well. But new features are being added and tested. To be sure that code will run across many platforms (don't forget the people who still use IE), many people have come to rely on transpilers/compilers. Transpilers/Compilers take 'dialects' of standard code (incuding ES6, coffeescript, typescript and more) and convert the code for you. A popular one for ES6 is Babel, they have both repl (Read, Evaluate, Print, Loop) and an npm module/gem.

Watch and Learn#

Note: As of 1/23/2017 All the following examples work in both node.js and Chrome.

IIFE#

IIFE - Immediately Invoked Function Expression - often used to create a block of scope. This is not an ES6 thing, but rather an example of an old way to get block scoping.

Now you can use let and const instead of var to get the same block scoping (see next section).

Thought question: var is the old way for declaring a variable. Why didn't standards get rid of it?

var a = 10;
(function IIFE() {  var a = 2;  console.log("Inside the IFFE, the value of a is", a); //"Inside the IFFE, the value of a is 2"})();
console.log("Outside the IFFE, the value of a is", a); //"Outside the IFFE, the value of a is 10s"
//Now let the world know that you know what an IIFE is! They'll be impressed!

Block Scoping#

A new way to get block level scoping is with the keyword let, and the use of {} as in any {}, not just tied to a function! Note: let is NOT meant to completely replace var!

var b = 1234;
{  let b = 5678;  console.log("the value of b inside the `{}` is", b); //"block level scope"}
console.log("the value of b outside of the `{}` is", b); //"global scope"

One more example

//a global variablevar j = 10;
/////////// ES5 /////////////get a little formatting, for clarityconsole.log("\n\nfor loop using `var`:\n");
//for loop using varfor (var iES5 = 0; iES5 < 5; iES5++) {  var kES5 = j + iES5;  console.log("the vale of iES5 inside the for loop", iES5);}
console.log("the value of iES5 outside the for loop is", iES5);console.log("the value of kES5 outside the for loop is", kES5);console.log("the value of j is", j);
/////////// ES6 ///////////
//get a little formatting, for clarityconsole.log("\n\nfor loop using `let`:\n");
//for loop using letfor (let iES6 = 0; iES6 < 5; iES6++) {  let kES6 = j + iES6;  console.log("the value of iES6 inside the for loop is ", iES6);}
console.log(  "the value of iES6 outside the for loop is: ReferenceError: iES6 is not defined"); // console.log(iES6) //error undefinedconsole.log("the value of j outside the for loop is", j);console.log(  "the value of kES6 outside the for loop is: `ReferenceError: kES6 is not defined`"); // console.log(kES6) //error undefined
//Woot woot!

Bonus#

Research on your own:

the following loop will not error:

for (z = 0; z < 5; z++) {  // do cool stuff}

Why is not including a let in front of the first z considered a poor practice? ``

Default values#

When creating a constructor and you wanted a default value, you previously had to write something like this:

const beverage_es5 = (type) => {  return type || "water";};
console.log(beverage_es5());console.log(beverage_es5("White Claw"));

Now you can do this in ES6

console.log("\n\n");
//Now you can do
const beverage_es6 = (type = "sparkling water") => {  return type;};
console.log(beverage_es6());console.log(beverage_es6("small beer"));
//Now that is refreshing!

Arrow Functions => => =>#

A new way to write a function expression (arrow functions are always anonymous functions), with a bonus feature!

More important than the new syntax, arrow functions bind the value of this

Let's compare the syntaxes:

// ES5 - 2 argumentsconst sumES5 = function (a, b) {  return a + b;};
console.log("\nES5 sum:", sumES5(5, 5));

If you have just one line of code, you can skip parenthesis and there is an implicit return (you don't need to write return to get a return value)

// ES6 - 2 argumentsconst sumES6 = (a, b) => a + b;
console.log("ES6 sum:", sumES6(6, 6));

//ES6 - no arguments, you can still skip the curlies if there is no argument and just one line of code. It is also ok if you don't have to return what the function is doing. In this case, we are calling console.log but we don't have to return a value

const salutationsES6 = () => console.log("Howdy ES6\n");
salutationsES6();
//ES5 - one argument

When there is just one argument, the parenthesis around the parameter is optional

//ES6 - one argumentvar squareES6 = c => c * c;console.log("ES6 square:", squareES6(6));

Round up: You can see {} and return are optional in short code blocks! Also, if there is just one argument, () is optional!

Binding this#

A function's scope is determined where the function is called. Not where the function is written. However, when you use an arrow function it will 'bring in' the value of this.

Let's see this in action

class Person {  constructor(    name,    friends = ["Charlie", "Dennis", "Ron", "Sweet Dee", "Frank"]  ) {    this.name = name;    this.friends = friends;  }  // undefined - can't run  secretAdmirer() {    this.friends.forEach(function (f) {      // console.log(this)      console.log(this.name + " sends flowers to " + f);    });  }
  // freeze the value of this by setting it to a new variable name  giveOldSchoolLove() {    const self = this;    // console.log(this)    // console.log(self)    this.friends.forEach(function (f) {      console.log(self.name + " likes " + f);    });  }  // bind the value of this  giveBindingAffection() {    // console.log(this)    this.friends.forEach(      function (f) {        // console.log(this)        console.log(this.name + " makes friendship bracelets for " + f);      }.bind(this)    );  }  // use an arrow function  giveNewSchoolLove() {    this.friends.forEach((f) => console.log(this.name + " hearts " + f));  }  allTheLove() {    // errors    // this.secretAdmirer()    // these all work    this.giveOldSchoolLove();    this.giveBindingAffection();    this.giveNewSchoolLove();  }}
//See examplesk = new Person("Karolin");console.log("\nSecret Admirer:");k.secretAdmirer();console.log("\nShow old school love:");k.giveOldSchoolLove();console.log("\nShow new school love:");k.giveNewSchoolLove();
// Supercalifragilisticexpialidocious !!!

Array.isArray()#

If you have tried to confirm if something is an array with ES5, you may have found it frustrating:

var arr = [1, 2, 3];
console.log(typeof arr); //'object'

You could do

arr instanceof Array // true

But now there is a new method!

Array.isArray(arr) //true
//Sweet!

Spread and Rest Operators#

The syntax is the same, but depending on what you are doing, it will act differently. The spread operator will take values and spread them into an array. The rest operator will gather values into an array

Spread operator:

//SPREAD (take out of array)//Let's look at an awesome Math methodconst x = -5;const y = 5;
console.log(Math.max(x, y)); // 5
// But what if you haveconst z = [1, 5, -2, 8, -9, 17, -22];// Using Math.max is doable, but cumbersome with an array.
// Now you can doconsole.log(Math.max(y, ...z)); // 17
// Easy as pie!

Rest operator:

//REST (gather into array)
//Let's get a bunch of arguments and sum the only the ones that are numbers
function sumOnlyNums(...args) {  var nums = args.filter((arg) => typeof arg === "number");  var sum = nums.reduce((sum, num) => sum + num);  return sum;}
console.log(  sumOnlyNums(    44,    false,    "pizza",    45,    { season: "winter" },    [1, 2, 3, 4, 5],    2,    9  )); // 100
// Duck soup FTW!

What's going on in the above function? What is nums? what is sum? Are they arrays? Numbers? Objects? What method could you call to check if one (or the other, or both) are arrays?

Also, those arrow functions look really nice, is there a this that they are binding or are they just there to make this code look cleaner? Play around with this function to learn!

Swap values: File under Destructuring Assignment#

//If you wanted to swap the value of x and y with es5,//you had to dolet x = true;let y = false;let temp;
temp = x;x = y;y = temp[  //The new way  //will return x and y to original values in this case  (x, y)] = [y, x];
// console.log(x, y)// NEATO!

Unpacking Objects: File under Destructuring Assignment#

One new trend in JS is to find ways to clean up the code. We can pull out values from an object

const someLongObjectName = {  brand: "Tandy",  price: 2894,  year: 1984,  ram: "128KB",  hardDrive: "20MB",  mouseIncluded: true,};

If we want to access the price, we always have to type the whole name of the object.

someLongObjectName.price;

However, now we can pull out the value

const { price } = someLongObjectName;
// console.log(price)
const { memory: ram } = someLongObjectName;
// renameconst { ram: memory } = someLongObjectName;console.log(memory);

We can also pull out more than one value

const { brand, year } = someLongObjectName;console.log(brand);console.log(year);

We can pull the nested object out:

const nestedObject = {  rabbit: {    hat: "rabbit",    rabbitCape: "fancy",  },};const tryToPullRabbitProperties = nestedObject.rabbit;const pullRabbitProperties = { ...nestedObject };console.log(tryToPullRabbitProperties);console.log(pullRabbitProperties);

Passed By Reference (arrays and objects)#

The way JavaScript stores arrays and objects in memory is it stores them in one place. You can add more pointers (variable names that access this object/array), but if you change the value, it changes everything. Therefore you have to be sure to copy your arrays/objects in the right way if you want to keep the original intact.

// Pass by referenceconst seasons = ["Spring", "Summer", "Fall", "Winter"];
console.log("seasons", seasons);
const reverseSeasons = seasons.reverse();
// make a copy using the spread operatorconst copySeason = [...seasons];
console.log("reverse seasons", reverseSeasons);
console.log("seasons", seasons);
reverseSeasons.reverse();
console.log("seasons", seasons);
console.log("copied season", copySeason);

Object literal upgrades#

//instead ofconst a = 8;const b = 9;const c = 10;
const es5obj = {  a: a,  b: b,  c: c,  d: function () {    console.log(es5obj.a, es5obj.b, es5obj.c);  },};
//now do
const es6obj = {  a,  b,  c,  d() {    console.log(es6obj.a, es6obj.b, es6obj.c);  },};
// What glorious syntactic sugar!

More More More#

New methods and features are being aded and tested all the time! Here are some great ones to look up and learn:

  • Async/Await: syntactic sugar for Promises!
  • Import/Export modules!
  • helper functions - trunc, entries, values, keys