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Modern JavaScript is evolving quickly to meet the changing needs of new frameworks and environments. Understanding how to take advantage of those changes can save you time, improve your skill set, and mark the difference between good code and great code.

Knowing what modern JavaScript is trying to do can help you decide when to use the new syntax to your best advantage, and when it still makes sense to use traditional techniques.

Something Solid to Cling To

I don’t know anybody who isn’t confused at the state of JavaScript these days, whether you’re new to JavaScript, or you’ve been coding with it for a while. So many new frameworks, so many changes to the language, and so many contexts to consider. It’s a wonder that anybody gets any work done, with all of the new things that we have to learn every month.

I believe that the secret to success with any programming language, no matter how complex the application, is getting back to the basics. If you want to understand Rails, start by working on your Ruby skills, and if you want to use immutables and unidirectional data flow in isomorphic React with webpack (or whatever the cool nerds are doing these days) start by knowing your core JavaScript.

Understanding how the language itself works is much more practical than familiarizing yourself with the latest frameworks and environments. Those change faster than the weather. And with JavaScript, we have a long history of thoughtful information online about how JavaScript was created and how to use it effectively.

The problem is that some of the new techniques that have come around with the latest versions of JavaScript make some of the old rules obsolete. But not all of them! Sometimes a new syntax may replace a clunkier one to accomplish the same task. Other times the new approach may seem like a simpler drop-in replacement for the way we used to do things, but there are subtle differences, and it’s important to be aware of what those are.

A Spoonful of Syntactic Sugar

A lot of the changes in JavaScript in recent years have been described as syntactic sugar for existing syntax. In many cases, the syntactic sugar can help the medicine go down for Java programmers learning how to work with JavaScript, or for the rest of us we just want a cleaner, simpler way to accomplish something we already knew how to do. Other changes seem to introduce magical new capabilities.

But if you try to use modern syntax to recreate a familiar old technique, or stick it in without understanding how it actually behaves, you run the risk of:

having to debug code that worked perfectly before

introducing subtle mistakes that may catch you at runtime

creating code that fails silently when you least expect it.

In fact, several of the changes that appear to be drop-in replacements for existing techniques actually behave differently from the code that they supposedly replace. In many cases, it can make more sense to use the original, older style to accomplish what you’re trying to do. Recognizing when that’s happening, and knowing how to make the choice, is critical to writing effective modern JavaScript.

When Your const Isn’t Consistent

Modern JavaScript introduced two new keywords, let and const , which effectively replace the need for var when declaring variables in most cases. But they don’t behave exactly the same way that var does.

In traditional JavaScript, it was always a clean coding practice to declare your variables with the var keyword before using them. Failure to do that meant that the variables you declared could be accessed in the global scope by any scripts that happened to run in the same context. And because traditional JavaScript was frequently run on webpages where multiple scripts might be loaded simultaneously, that meant that it was possible for variables declared in one script to leak into another.

The cleanest drop-in replacement for var in modern JavaScript is let . But let has a few idiosyncrasies that distinguish it from var . Variable declarations with var were always hoisted to the top of their containing scope by default, regardless of where they were placed inside of that scope. That meant that even a deeply nested variable could be considered declared and available right from the beginning of its containing scope. The same is not true of let or const .

console . log ( usingVar ) ; var usingVar = "defined" ; console . log ( usingVar ) ; console . log ( usingLet ) ; let usingLet = "defined" ; console . log ( usingLet ) ;

When you declare a variable using let or const , the scope for that variable is limited to the local block where it’s declared. A block in JavaScript is distinguished by a set of curly braces {} , such as the body of a function or the executable code within a loop.

This is a great convenience for block-scoped uses of variables such as iterators and loops. Previously, variables declared within loops would be available to the containing scope, leading to potential confusion when multiple counters might use the same variable name. However let can catch you by surprise if you expect your variable declared somewhere inside of one block of your script to be available elsewhere.

for ( var count = 0 ; count < 5 ; count ++ ) { console . log ( count ) ; } console . log ( count ) ; for ( let otherCount = 0 ; otherCount < 5 ; otherCount ++ ) { console . log ( otherCount ) ; } console . log ( otherCount ) ;

The other alternative declaration is const , which is supposed to represent a constant. But it’s not completely constant.

A const can’t be declared without a value, unlike a var or let variable.

var x ; let y ; const z ;

A const will also throw an error if you try to set it to a new value after it’s been declared:

const z = 3 ; z = 4 ;

But if you expect your const to be immutable in all cases, you may be in for a surprise when an object or an array declared as a const lets you alter its content.

const z = [ ] ; z . push ( 1 ) ; z = [ 2 ]

For this reason, I remain skeptical when people recommend using const constantly in place of var for all variable declarations, even when you have every intention of never altering them after they’ve been declared.

While it’s a good practice to treat your variables as immutable, JavaScript won’t enforce that for the contents of a reference variables like arrays and objects declared with const without some help from external scripts. So the const keyword may make casual readers and newcomers to JavaScript expect more protection than it actually provides.

I’m inclined to use const for simple number or string variables I want to initialize and never alter, or for named functions and classes that I expect to define once and then leave closed for modification. Otherwise, I use let for most variable declarations — especially those I want to be bounded by the scope in which they were defined. I haven’t found the need to use var lately, but if I wanted a declaration to break scope and get hoisted to the top of my script, that’s how I would do it.

Limiting the Scope of the Function

Traditional functions, defined using the function keyword, can be called to execute a series of statements defined within a block on any parameters passed in, and optionally return a value:

function doSomething ( param ) { return ( ` Did it: ${ param } ` ) ; } console . log ( doSomething ( "Hello" ) ) ;

They can also be used with the new keyword to construct objects with prototypal inheritance, and that definition can be placed anywhere in the scope where they might be called:

function Animal ( name ) { this . name = name ; } let cat = new Animal ( "Fluffy" ) ; console . log ( ` My cat's name is ${ cat . name } . ` ) ;

Functions used in either of these ways can be defined before or after they’re called. It doesn’t matter to JavaScript.

console . log ( doSomething ( "Hello" ) ) ; let cat = new Animal ( "Fluffy" ) ; console . log ( ` My cat's name is ${ cat . name } . ` ) ; function doSomething ( param ) { return ( ` Did it: ${ param } ` ) ; } function Animal ( name ) { this . name = name ; }

A traditional function also creates its own context, defining a value for this that exists only within the scope of the statement body. Any statements or sub-functions defined within it are executing, and optionally allowing us to bind a value for this when calling the function.

That’s a lot for the keyword to do, and it’s usually more than a programmer needs in any one place. So modern JavaScript split out the behavior of the traditional function into arrow functions and classes.

Getting to Class on Time

One part of the traditional function has been taken over by the class keyword. This allows programmers to choose whether they would prefer to follow a more functional programming paradigm with callable arrow functions, or use a more object-oriented approach with classes to substitute for the prototypal inheritance of traditional functions.

Classes in JavaScript look and act a lot like simple classes in other object-oriented languages, and may be an easy stepping stone for Java and C++ developers looking to expand into JavaScript as JavaScript expands out to the server.

One difference between functions and classes when doing object-oriented programming in JavaScript is that classes in JavaScript require forward declaration, the way they do in C++ (although not in Java). That is, a class needs to be declared in the script before it is instantiated with a new keyword. Prototypal inheritance using the function keyword works in JavaScript even if it’s defined later in the script, since a function declaration is automatically hoisted to the top, unlike a class .

let aProto = new Proto ( "Myra" ) ; aProto . greet ( ) ; function Proto ( name ) { this . name = name ; this . greet = function ( ) { console . log ( ` Hi ${ this . name } ` ) ; } ; } ; class Classy { constructor ( name ) { this . name = name ; } greet ( ) { console . log ( ` Hi ${ this . name } ` ) ; } } ; let aClassy = new Classy ( "Sonja" ) ; aClassy . greet ( ) ;

Pointed Differences with Arrow Functions

The other aspect of traditional functions can now be accessed using arrow functions, a new syntax that allows you to write a callable function more concisely, to fit more neatly inside a callback. In fact, the simplest syntax for an arrow function is a single line that leaves off the curly braces entirely, and automatically returns the result of the statement executed:

const traditional = function ( data ) { return ( ` ${ data } from a traditional function ` ) ; } const arrow = data => ` ${ data } from an arrow function ` ; console . log ( traditional ( "Hi" ) ) ; console . log ( arrow ( "Hi" ) ) ;

Arrow functions encapsulate several qualities that can make calling them more convenient, and leave out other behavior that isn’t as useful when calling a function. They are not drop-in replacements for the more versatile traditional function keyword.

For example, an arrow function inherits both this and arguments from the contexts in which it’s called. That’s great for situations like event handling or setTimeout when a programmer frequently wants the behavior being called to apply to the context in which is it was requested. Traditional functions have forced programmers to write convoluted code that binds a function to an existing this by using .bind(this) . None of that is necessary with arrow functions.

class GreeterTraditional { constructor ( ) { this . name = "Joe" ; } greet ( ) { setTimeout ( function ( ) { console . log ( ` Hello ${ this . name } ` ) ; } , 1000 ) ; } } let greeterTraditional = new GreeterTraditional ( ) ; greeterTraditional . greet ( ) ; class GreeterBound { constructor ( ) { this . name = "Steven" ; } greet ( ) { setTimeout ( function ( ) { console . log ( ` Hello ${ this . name } ` ) ; } . bind ( this ) , 1000 ) ; } } let greeterBound = new GreeterBound ( ) ; greeterBound . greet ( ) ; class GreeterArrow { constructor ( ) { this . name = "Ravi" ; } greet ( ) { setTimeout ( ( ) => { console . log ( ` Hello ${ this . name } ` ) ; } , 1000 ) ; } } let greeterArrow = new GreeterArrow ( ) ; greeterArrow . greet ( ) ;

Understand What You’re Getting

It’s not all just syntactic sugar. A lot of the new changes in JavaScript have been introduced because new functionality was needed. But that doesn’t mean that the old reasons for JavaScript’s traditional syntax have gone away. Often it makes sense to continue using the traditional JavaScript syntax, and sometimes using the new syntax can make your code much faster to write and easier to understand.

Check out those online tutorials you’re following. If the writer is using var to initialize all of the variables, ignoring classes in favor of prototypal inheritance, or relying on function statements in callbacks, you can expect the rest of the syntax to be based on older, traditional JavaScript. And that’s fine. There’s still a lot that we can learn and apply today from the traditional ways the JavaScript has always been taught and used. But if you see let and const in the intializations, arrow functions in callbacks, and classes as the basis for object-oriented patterns, you’ll probably also see other modern JavaScript code in the examples.

The best practice in modern JavaScript is paying attention to what the language is actually doing. Depending on what you’re used to, it may not always be obvious. But think about what the code you’re writing is trying to accomplish, where you’re going to need to deploy it, and who will be modifying it next. Then decide for yourself what the best approach would be.