Embrace modern JavaScript - ES6 and beyond (part 1)


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JavaScript used to be considered one of those "lesser" languages, kind of like PHP, that were deemed unsuitable for "real" programming. After all, yes, JavaScript is dynamically typed (the type of a variable is known/can vary at runtime) and is indecently permissive/forgiving. Here are a few things that you can do without having runtime errors:

  • add values of different types (such as numbers and strings),
  • use a variable that has not been assigned before,
  • reference non-existent properties of an object,
  • call a function with less or more arguments than it expects.

This works because JavaScript has implicit type conversion, so it tries to make sense of what it is supposed to do when computing 1 + '2' for instance. Also, JavaScript has an undefined type, which is the type of an expression that is, well, not defined, such as accessing a non-existent property or a missing argument.

JavaScript has been an international standard since 1997, when it was first standardized as ECMAScript. There are several editions of ECMAScript, and the nth edition of ECMAScript is generally abbreviated as ESn: ES6 means the sixth edition (published in 2015). We will explore in a series of playgrounds the modern features of the language that will make your code more robust, concise, and easier to read.

In this playground, you will read about:

The not-so-distant past (ES5)

Strict mode

Starting with ES5 (2009), JavaScript has a strict mode that does just what it says, i.e. it disallows things that are considered too lenient. Among other things, strict mode:

  • removes legacy octal notation for numbers and octal escape sequences in strings,
  • restricts the use of eval and runs the evaluated code in an isolated environment,
  • restricts the use of arguments and disables caller/callee properties,
  • makes the with statement a syntax error,
  • forbids having multiple properties/arguments with the same name,
  • no longer coerces this to the global object if it is null or undefined.

This last point is shown below:

TL;DR you should always 'use strict' in your code to make it cleaner by not using what are mostly legacy features that have cleaner replacements.

Higher order functions

ES5 also added higher-order functions to arrays. This means that if you wanted to create a new array from another array with values incremented, instead of doing it the old way:

You could abstract the iteration away and create a new array using the map function:

We will see later in this playground how we can further improve this example.

The present (ES6)

With the release of ES6 (2015), JavaScript really caught up with other programming languages. This version introduced many changes and new features that make JavaScript far more powerful, while also fixing a few long-standing issues. ES6 is now supported natively in the four major browsers: Chrome, Edge, Firefox, Safari. The only mainstream browser not supporting ES6 is... Internet Explorer 11, (unfortunately) still maintained by Microsoft to this day.

Use let and const, not var

There are two problems with var. First, it has function scope even if it is declared in a nested block. This means that outside the for loop below, you can still use the variable i.

This prints 3. Change var to let and observe the difference.

The second problem is that within a function two declarations of a var x will in fact be the same variable x. For instance:

What does this print? Try to replace var by let and see what happens. Move the let x = 4; statement before the if, and run the code again.

Note how you are allowed to declare a different variable with the same name as long as it is not in the same scope.

const behaves similarly to let except that it does not let you assign a different value to the variable. This is referred to as an immutable binding: the binding (association of the value to the identifier) is not mutable, though the value itself is, as illustrated by the code below:

Use arrow functions instead of anonymous functions

As you already know, JavaScript has long had first-class support for functions: you can pass functions as arguments to other functions and create functions dynamically. However, the function () syntax can feel a bit clunky, and so-called arrow functions are a nice improvement in syntax. More than that though, by reducing visual burden, they also enable a way of thinking that is traditionally found in functional programming languages.

Our ES5 code for mapping an array by incrementing all values was:

Compare this to the ES6 version:

An arrow function can either return an expression directly (as is the case here), or have a normal body with statements if it begins with {. This means that if you want to return an object directly you should wrap it in parentheses. An arrow function with 0 parameters or more than 1 parameter must have a list of parameters in parentheses. For example an add function defined in ES5 as follows:

var add = function(x, y) {
    return x + y;

can be written in ES6 with an arrow function as:

Note the use of const here to prevent the add variable from being reassigned. Interestingly, with arrow functions it becomes much more feasible to create functions in curried form, i.e. to transform a function taking multiple arguments into multiple nested functions taking one argument each, with the innermost function doing the actual computation. Currying the add function leads to:

In ES5:

var add = function(x) {
    return function(y) {
        return x + y;

In ES6:

The advantage of working with curried functions is that you can use partial application. This means supplying the function with fewer arguments than necessary. We use partial application to increment elements with add as follows:

This pattern is used in practice, for instance a middleware in Redux will look like this:

function myMiddleware() {
    return next => action => {
        // do something with action
        // ...

        // Call the next dispatch method in the middleware chain.
        return next(action)

Arrow functions and this

Another advantage of arrow functions is that they keep the existing value of this. For instance, the following ES5 code does not work:

That's because in strict mode, this is actually undefined in the inner function. There are several workarounds: you could assign this to a self variable before the call to forEach, you could pass this as an additional argument to forEach, you could bind the inner function to this, or you can just use an arrow function in ES6:

Note how we used an anonymous function for computeSum rather than an arrow function so that we have the proper this.

Alternatives to arguments

One of JavaScript's great strengths is its flexibility when calling functions. To the caller, a function signature is mostly indicative: you can call a function with fewer or more arguments than it declares, and it will work fine. In fact it enables interesting APIs with functions that do different things depending on the number and types of arguments they are given.

The problem is that until ES6 you often had to manipulate the special arguments variable for that, even in simple cases. ES6 adds two features that let you specify function signatures stating your intent more clearly and often let you get away without using arguments.

Default parameters

Optional arguments have been supported since day one in JavaScript, but before ES6 they required boilerplate code such as:

Now you can simply use default parameters instead:

The default value of a parameter can be another parameter that is declared before it:

Rest parameters

Default parameters work well for functions with a fixed number of parameters, but for functions that accept a variable number of parameters you need rest parameters.

For instance, assume you want to sum numbers with an initial number (optional, default to zero), in ES5 you would write something like this:

Note that you cannot call slice directly on arguments because it is not a real Array object.

Now with ES6 this code becomes:

The ...values rest parameter must be the last parameter of the function (or the only one). It is an array of arguments that were given to the function after preceding parameters (if any). The ... operator is called the spread operator. We will see in part 2 more advanced use of rest parameters with destructuring.

Template literals

Template literals are basically super strings. The primary use of template literals is to create a string from a template string and expressions that are evaluated and concatenated together. A template literal can also contain newlines without the need to escape them. Compare the following examples:

String concatenation in a template string behaves like String.prototype.concat rather than the + operator. This means that in a template literal, an expression is converted to a string using toString (and if not present, valueOf), whereas with the + operator the order is reversed.

Try to remove valueOf or toString and run the code again to see the differences.

What's next?

In part 2, we will see improved object literals, the for-of loop, destructuring in declarations and assignements, and talk more about using the ... spread operator.

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