JavaScript

JavaScript (or ECMAScript, which is the standard JavaScript is derived from), was originally developed for Netscape Navigator by Brendan Eich. The original version was completed in just 10 days. The name “JavaScript” was a marketing move by Netscape as they had just secured the rights to use Java Applets in their browser, and wanted to tie the two languages together. Similarly, they pushed for a Java-like syntax, which Brendan accommodated. However, he also incorporated functional behaviors based on the Scheme language, and drew upon Self’s implementation of object-orientation. The result is a language that may look familiar to you, but often works in unexpected ways.

JavaScript is a Dynamically Typed Language

Unlike the statically-typed Java language, JavaScript has dynamic types like Python. This means that we always declare variables using the var keyword, i.e.:

var i = 0;
var story = "Jack and Jill went up a hill...";
var pi = 3.14;

The type of the variable is inferred when it is set, and the type can change with a new assignment, i.e.:

var i = 0; // i is an integer
i = "The sky is blue"; // now i is a string
i = true; // now i is a boolean

This would cause an error in C#, but is perfectly legal in JavaScript. Because JavaScript is dynamically typed, it is impossible to determine type errors until the program is run.

In addition to var, variables can be declared with the const keyword (for constants that cannot be re-assigned), or the let keyword (discussed below).

JavaScript Types

While the type of a variable is inferred, JavaScript still supports types. You can determine the type of a variable with the typeof() function. The available types in JavaScript are:

• integers (declared as numbers without a decimal point)
• floats (declared as numbers with a decimal point)
• booleans (the constants true or false)
• strings (declared using double quotes ("I'm a string"), single quotes 'Me too!', or backticks `I'm a template string ${2 + 3}`) which indicate a template string and can execute and concatenate embedded JavaScript expressions.
• lists (declared using square brackets, i.e. ["I am", 2, "listy", 4, "u"]), which are a generic catch-all data structure, which can be treated as an array, list, queue, or stack.
• objects (declared using curly braces or constructed with the new keyword, discussed later)

In JavaScript, there are two keywords that represent a null value, undefined and null. These have a different meaning: undefined refers to values that have not yet been initialized, while null must be explicitly set by the programmer (and thus intentionally meaning nothing).

JavaScript is a Functional Language

As suggested in the description, JavaScript is a functional language incorporating many ideas from Scheme. In JavaScript we declare functions using the function keyword, i.e.:

function add(a, b) {
  return a + b;
}

We can also declare an anonymous function (one without a name):

function (a, b) {
  return a + b;
}

or with the lambda syntax:

(a,b) => {
  return a + b;
}

In JavaScript, functions are first-class objects, which means they can be stored as variables, i.e.:

var add = function(a,b) {
  return a + b;
}

Added to arrays:

var math = [
  add,
  (a,b) => {return a - b;},
  function(a,b) { a * b; },
]

Or passed as function arguments.

JavaScript has Function Scope

Variable scope in JavaScript is bound to functions. Blocks like the body of an if or for loop do not declare a new scope. Thus, this code:

for(var i = 0; i < 3; i++;)
{
  console.log("Counting i=" + i);
}
console.log("Final value of i is: " + i);

Will print:

Counting i=0
Counting i=1
Counting i=2
Final value of i is: 3

Because the i variable is not scoped to the block of the for loop, but rather, the function that contains it.

The keyword let was introduced in ECMAScript version 6 as an alternative for var that enforces block scope. Using let in the example above would result in a reference error being thrown, as i is not defined outside of the for loop block.

JavaScript is Event-Driven

JavaScript was written to run within the browser, and was therefore event-driven from the start. It uses the event loop and queue pattern we saw in C#. For example, we can set an event to occur in the future with setTimeout():

setTimeout(function(){console.log("Hello, future!")}, 2000);

This will cause “Hello, Future!” to be printed 2 seconds (2000 milliseconds) in the future (notice too that we can pass a function to a function).

JavaScript is Object-Oriented

As suggested above, JavaScript is object-oriented, but in a manner more similar to Self than to C#. For example, we can declare objects literally:

var student = {
  first: "Mark",
  last: "Delaney"
}

Or we can write a constructor, which in JavaScript is simply a function we capitalize by convention:

function Student(first, last){
  this.first = first;
  this.last = last;
}

And invoke with the new keyword:

var js = new Student("Jack", "Sprat");

Objects constructed from classes have a prototype, which can be used to attach methods:

Student.prototype.greet = function(){
  console.log(`Hello, my name is ${this.first} ${this.last}`);
}

Thus, js.greet() would print Hello, my name is Jack Sprat;

ECMAScript 6 introduced a more familiar form of class definition:

class Student{
  constructor(first, last) {
    this.first = first;
    this.last = last;
    this.greet = this.greet.bind(this);
  }
  greet(){
    console.log(`Hello, my name is ${this.first} ${this.last}`);
  }
}

However, because JavaScript uses function scope, the this in the method greet would not refer to the student constructed in the constructor, but the greet() method itself. The constructor line this.greet = this.greet.bind(this); fixes that issue by binding the greet() method to the this of the constructor.

The Document Object Model

The Document Object Model (DOM) is a tree-like structure that the browser constructs from parsed HTML to determine size, placement, and appearance of the elements on-screen. In this, it is much like the elements tree we used with Windows Presentation Foundation (which was most likely inspired by the DOM). The DOM is also accessible to JavaScript - in fact, one of the most important uses of JavaScript is to manipulate the DOM.

You can learn more about the DOM from MDN’s Document Object Model documentation entry.