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[fit] JavaScript / TypeScript

[fit] What is it? (JavaScript)


  • Initially to make web pages "interactive".
  • Is the lingua franca of the front-end web, the default programming language.
  • Has extended to the server-side via implementations like Node.js
  • Showing up in more places like embedded environments.


  • Created around 1995.
  • JavaScript is not Java.
  • Became a standard in 1997.


  • Draws from Java, C, Scheme.
  • Some syntax will look familiar since Java and C also inspire C#.

Also statement based

Like C# a TypeScript program is composed of a sequence of statements.

Like a C# program, these statements may end with a semicolon, ;. However, these are optional in TypeScript, and we will code without them.

Comments are the same as in C#, single lines with // and multiple lines with /* and */.

Typed language

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JavaScript also has types, some of which will be familiar.

  • number
  • string
  • boolean
  • array
  • object
  • null
  • undefined

But JavaScript is loosely typed!

let score = 42
// WHAT? This ok in JavaScript!?
score = 'Mary'



Where are loose types helpful?

JavaScript will allow us to easily mix types

const arrayOfNumbersAndStrings = [
const people = [
{ name: 'Susan' },
{ name: 'Mary', partTime: true },
{ name: 'Frank', salary: 42 },

JavaScript had a bad rap for being too flexible, leading to bad code

And there is a lot of it...


It is a JavaScript standard meant to ensure the interoperability of web pages across different web browsers.

Starting in 2015 we've been numbering them by year. (you may still see version numbers used)

ECMAScript has been adding features making JavaScript better!

ECMAScript improvements has displaced alternative languages like CoffeeScript that would compile to JavaScript.

While JavaScript has been getting better, it still lacked a good way of managing data types.

Calling for types in JavaScript ... Enter TypeScript

Developed at Microsoft. First released around 2012, started becoming very popular in the last few years.

Around 2018 the language and the tooling started becoming good enough for mass adoption.

See: this artcile

[fit] So why is TypeScript getting popular?

  • Rise of the JavaScript Framework + Library

    • React
    • Angular
    • Svelte
    • Vue
  • These allow us to organize and build LARGE front end applications

More code, more problems

Large applications split among many developers are difficult to manage.

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Can types help?

TypeScript adds additional syntax to JavaScript to support a tighter integration with your editor. Catch errors early in your editor.

TypeScript code converts to JavaScript which runs anywhere JavaScript runs: In a browser, on Node.js or Deno and in your apps.

TypeScript understands JavaScript and uses type inference to give you great tooling without additional code.


TypeScript is a SUPER SET of JavaScript

  • Everything that is JavaScript is also TypeScript (well, almost, and the parts that are not aren't important to us now)

  • TypeScript compiles down to JavaScript that our browser can understand (mostly by removing all the extra syntax we'll see in a moment)

  • Projects can adopt TypeScript gradually

[fit] TypeScript adds a flexible type system

  • We can declare types for all our variables

  • We can declare our own types!

Can create our own types!

During this lecture, and in our work, we'll create our own types!

See some examples

Literal values

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Like C# we can declare literal values.

  • Numbers: 0, 117 and -345 but also 3.1415 and -42.123
  • Strings: "This is a sentence" 'This is a sentence' `This is a sentence`
  • Boolean: true and false
  • Arrays: ["French Roast", "Columbian", "Kona"] and ["Shuttle", 42, true, "Thing"]
  • Objects: {name: "Sandy", enrolled: true}


Must start with a letter (upper or lower), an underscore, or a dollar sign. Variable names may also have digits in the name after the first character.

Standard to use camelCase for our variable names.


Four ways to declare a variable

varDeclares a variable to be function or globally scoped, optionally initializing it to a value.
letDeclares a block-scoped, local variable, optionally initializing it to a value.
constDeclares a block-scoped, read-only named constant.
undeclared globalWithout var, let, or const, we create a global variable

Our style

undeclared global variables are highly discouraged as they can often lead to unexpected behavior.

We will always use let or const in our coding.

When looking at code on the web, e.g., StackOverflow, and blog posts, you will still see var.


const variables are assigned a value on the same statement where they are declared.

They can also not be re-assigned later in the code.

const answer = 42

Not allowed, will be an error:

answer = answer + 1


let variables do not need to be assigned a value on the same statement they are declared.

They can be re-assigned later in the code.

let score = 98
score = score + 1


let and const are considered block scope.

That is, they are valid and accessible inside the current block. A block is:

  • A pair of { } which we will see for functions and statements such as if, while, switch, and others.
  • The source file they are in if there is no current block.

So, what about types?

  • Shouldn't we be declaring types for our variables?
  • Like C#, TypeScript has type inference.

So the following code declares variables that are of type number!

const answer = 42
let score = 98

TypeScript inference is quite good!

const name = 'Mary'
const students = ['Mary', 'Steven', 'Paulo', 'Sophia']
const scores = [98, 100, 55, 100]
  • name is a string
  • students is an array of strings
  • scores is an array of numbers

We can see this over in Visual Studio if we have a TypeScript file!

Mix and Match

What about an array that has different types of elements?

Unlike C#, TypeScript can handle that just fine, and in a very nice way.

fit right

Mix and Match

const differentKindsOfThings = [42, 'Ice Cream', 100, 'Tacos']
The variable differentKindsOfThings
will have the type
(string | number)[]

The | is a union of types; it means or.

differentKindsOfThings is an array of elements that can be either a string or a number.

Declaring types explicitly

const name: string = 'Mary'
const students: string[] = ['Mary', 'Steven', 'Paulo', 'Sophia']
const scores: number[] = [98, 100, 55, 100]
const differentKindsOfThings: (string | number)[] = [
'Ice Cream',

Declaring types explicitly

Once we introduce the idea of TypeScript objects we'll discuss why specifying an explict type is useful.

Undefined variables

After declaring a variable but before assigning it a value, the variable will contain a special but confusing, value known as undefined

The type of the variable will also be any which means it will accept a value of any kind.

let name // name contains 'undefined', and is of `any` type
name = 'Jane' // name now contains the value 'Jane'

Avoid undefined and any in your code!

In fact, we can turn on code checking tools to make sure we avoid them!

Bad form

let name
// name contains 'undefined' and is of type `any`
name = 'Jane'
// name now contains the value 'Jane', but `name` is still an `any` type.

Better form

let name: string
// name contains 'undefined' and should be of type `string`
name = 'Jane'
// name now contains the value 'Jane'

Best form

const name = 'Jane'
// name contains the value 'Jane' and we avoid any issue with `undefined`

Good variable hygiene

Follow these rules, and you'll do well:

  • Only use const and let.
  • Use const unless you have a good reason to use let
  • Always initialize a variable unless you have a good reason not to.


TypeScript is far more forgiving when converting types.

Valid in TypeScript but not allowed in a language like C#

const x = 'The answer is ' + 42 // "The answer is 42"
const y = 42 + ' is the answer' // "42 is the answer"

Amazing, right?

... not so fast!

Doesn't work as you might expect

In statements involving other operators, TypeScript does not convert numeric values to strings. For example:

let x: string
x = '37' - 7 // 30 and notes an error
x = '37' + 7 // "377" does not note any error

String interpolation

const score = 98
const answer = 42
const message = `Congratulations, ${answer} is correct. You have ${score} points`


Similar to other languages objects are a combination of state and behavior.

In TypeScript, an object is a standalone entity with properties and type. Compare it with a cup, for example. A cup is an object with properties. A cup has a color, a design, weight, and a material. In the same way, TypeScript objects can have properties, which define their characteristics.

Object Properties

A TypeScript object has properties associated with it.

A property of an object is a variable that is attached to the object.

Object properties are the same as ordinary TypeScript variables, except for the attachment to objects.

The properties of an object define the characteristics of the object.

Object Properties

You access the properties of an object with a simple dot-notation:


Object Properties

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Like all TypeScript variables, both the object name (which could be a standard variable) and property name are case-sensitive.

You can define a property by assigning it a value.

For example, let's create an object named myCar and give it properties named make, model, and year as follows:


const myCar = {
make: 'Ford',
model: 'Mustang',
year: 1969,

Object Initializer

The previous example uses an object initializer, which is a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}):

Accessing properties using bracket notation (and by string)


Properties of TypeScript objects can also be accessed or set using a bracket notation.

So, for example, you could access the properties of the myCar object as follows:


myCar['make'] = 'Ford'
myCar['model'] = 'Mustang'
myCar['year'] = 1969


An object property name can be any valid TypeScript string or anything convertible into a string, including the empty string.

However, any property name that is not a valid TypeScript identifier (for example, a property name with space or hyphens, or that starts with a number) can only use the square bracket notation.

Better form (best practice)

myCar.make = 'Ford'
myCar.model = 'Mustang'
myCar.year = 1969

Make a second car

const theirCar = {
make: 'Jeep',
model: 'Wrangler',
year: 2021,

This car has the same type as the myCar variable, but only through coincidence.

Make another car

const otherCar = {
make: 'Honda',
modal: 'Fit',
year: 2020,

Anyone see a problem?


const otherCar = {
make: 'Honda',
modal: 'Fit',
year: 2020,

I make many typos and similar errors. Maybe TypeScript can help me?

[fit] Using types to find issues in our code

We can teach TypeScript about a new specific type and give it a name of our choice.

type Car = {
make: string
model: string
year: number

Using the type


type Car = {
make: string
model: string
year: number
const myCar: Car = {
make: 'Ford',
model: 'Mustang',
year: 1969,


const theirCar: Car = {
make: 'Jeep',
model: 'Wrangler',
year: 2021,
const otherCar: Car = {
make: 'Honda',
modal: 'Fit',
year: 2020,

[fit] The modal error will be called out

[fit] Making a new object from an existing one


Let's take our car example again:

const myCar = {
make: 'Ford',
model: 'Mustang',
year: 1969,


Make a new car, but with a different year.

const myOtherCar = {
make: myCar.make,
model: myCar.model,
year: 1971,

[fit] Fortunately, TypeScript allows for a shortcut

  • "expands" all the keys and values.
  • This is known as the spread operator and is noted as ...
const myOtherCar = {
year: 1971,

P.S. Up-to-date versions of JavaScript have this too...


TypeScript also has an array type. TypeScript arrays are more flexible than C#'s and are more akin to List<> in their flexibility. TypeScript arrays also differ from C# arrays as TypeScript arrays can store values of different types.

There are three ways to declare an array:

const array = new Array(element0, element1, ..., elementN);
const array = Array(element0, element1, ..., elementN);
const array = [element0, element1, ..., elementN];

[fit] Array and new Array are problematic

  • const array = new Array('hello', 42) will define an array of string and convert the 42 to a string.
  • const array = new Array(42, 'hello') will be a TypeScript error.
  • These are not recommended approaches for creating arrays.

Array literals

const people = ['Betty', 'Wilma', 'Fred', 'Barny']
const scores = [100, 42, 50, 98]
const collection = ['Betty', 98, 'Fred', 12, 42]

Even though the arrays contain different types, TypeScript will create the correct kind of array.

Populating an array

We can also use the [] operator to assign values to specific elements of an array.

const employees = []
employees[0] = 'Rivest, Ron'
employees[1] = 'Shamir, Adi'
employees[2] = 'Adleman, Leonard'

However, employees will be an array of any!

This is better

const employees: string[] = []
employees[0] = 'Rivest, Ron'
employees[1] = 'Shamir, Adi'
employees[2] = 'Adleman, Leonard'

This is better yet

const employees = ['Rivest, Ron', 'Shamir, Adi', 'Adleman, Leonard']

Wait, what about const?

const only refers to the variable, not it's contents.

It only prevents us from doing

employees = ['Peter', 'Paul', 'Mary']

This is totally valid:


[fit] Can you make an unchangable array?

  • Yes, use ReadOnlyArray<>
const cantChangeTheseValues: ReadonlyArray<number> = [42, 100, 52]
cantChangeTheseValues[0] = 1

[fit] Editor/Compile time versus Runtime

Editor is screaming at us, browser might show errors, but...


TypeScript transforms down to JavaScript so while it will complain and warn, it won't STOP you.

We can turn on some project configurations to not allow our code to run until it is free of these kinds of warnings


Iterating arrays

A common operation is to iterate over the values of an array, processing each one in some way. The simplest way to do this is as follows:

let colors = ['red', 'green', 'blue']
for (let index = 0; index < colors.length; index++) {

Iterating arrays

The forEach() method provides another way of iterating over an array:

let colors = ['red', 'green', 'blue']
colors.forEach(function (color) {

[fit] Iterating arrays with arrow functions

Alternatively, You can shorten the code for the forEach parameter with Arrow Functions:

let colors = ['red', 'green', 'blue']
colors.forEach(color => console.log(color))

Looks similar to C#, eh? -- more about this tomorrow...

More about iterating

For more details on how to manipulate arrays, including adding and removing elements, see the quick reference guide on arrays.

Control flow

Control flow in TypeScript is nearly identical to C# in that our code consists of a sequential set of statements that comprises a block of code:



We can control the flow of the code with a conditional statement:

if (condition) {
} else {

This works exactly as C# except for the different style in how the braces and else are formatted.


The same boolean comparisons are present in TypeScript, <, >, <=, >=, and ==.

const answer = 42
const score = 98
if (answer < 43) {
// statement
if (score > 97) {
// statement
if (score == 98) {


== doesn't work as you might expect

It doesn't mean is exactly as we've seen in a language like C#

It means is this thing kinda like the other thing?

It first sees if the values can be converted to a common type and then performs the comparison.


const answer = 42
const message = '42'
if (answer == message) {
// Yup! this will be *TRUE*
// However, TypeScript will complain at us!

== rules


^ Nobody wants to memorize these ^ I haven't memorized them

So, what do we do?

Enter the threequal (===)

In TypeScript

==Abstract Equality Comparison
===Strict Equality Comparison

=== is typically what we want

In most cases === is what you want when comparing values.

There are some exceptions, but they are very rare.

Want more?

See this article

For loop

for(let index = 0; index < 20; index++) {
console.log(`The index is ${index})


switch (expression) {
case label_1:
case label_2:

This should seem familiar


In TypeScript, named groups of code that perform a specific behavior are called functions (whereas in C# we called them methods).

Also, these functions do not need to be in classes.


The following code defines a simple function named greet:

// function keyword
// |
// | name of the function
// | |
// | | required parenthesis where arguments will go
// | | |
// | | | opening scope of the function
// | | | |
// | | | |
// v v v v
function greet() {
console.log('Hello there programmer!')

Add arguments

For example, the following code defines a simple function named square.

// argument type
// |
// | function return type (optional)
// | |
// | |
// v v
function square(valueToSquare: number): number {
return valueToSquare * valueToSquare

Calling functions

const answer = square(5)

If there is one way, more ways are better!

[fit] Function Expressions

Function expressions

While the function declaration above is syntactically a statement, functions can also use a function expression style.

const square = function (valueToSquare: number) {
return valueToSquare * valueToSquare
const answer = square(4) // answer gets the value 16

[fit] Functions are just another kind of type!

We say that functions in TypeScript are a type just like numbers, strings and booleans.

We can assign them names and pass them as arguments.

type PrintItFunction = (value: number) => number
function printIt(numbers: number[], func: PrintItFunction) {
for (let index = 0; index < numbers.length; index++) {
const value = numbers[index]
const result = func(value)
console.log(`Turned ${value} into ${result}`)

function square(valueToSquare: number) {
return valueToSquare \* valueToSquare
function double(valueToDouble: number) {
return valueToDouble * 2
const numbers = [1, 2, 3, 4, 5]
printIt(numbers, square)
// Turned 1 into 1
// Turned 2 into 4
// Turned 3 into 9
// Turned 4 into 16
// Turned 5 into 25
printIt(numbers, double)
// Turned 1 into 2
// Turned 2 into 4
// Turned 3 into 6
// Turned 4 into 8
// Turned 5 into 10

Here is where TypeScript shines!

What if we define a new function that doesn't fit the pattern our printIt function expects.

function upperCase(stringToUpperCase: string) {
return stringToUpperCase.toUpperCase()
const words = ['hello', 'there']
printIt(words, upperCase)

The TypeScript system would immediately tell us that words isn't an array of numbers and cannot be sent to printIt!


If we "fix" this error by using our numbers variable, we'll see that TypeScript then notifies us that the upperCase doesn't follow the style of the function we are expecting!



Passing functions as arguments to other functions is a very powerful pattern in TypeScript. We will be using this ability quite a bit in other lessons.

Functions treated as values for variables and passed as arguments are two things that make TypeScript a functional-style language.

Scope again

We cannot access variables defined inside a function from anywhere outside the function because the variable is defined only in the function's scope.

However, a function can access all variables and functions defined inside its scope.

A function defined inside another function can also access all variables defined in its parent function and any other variable to which it has access.

const PI = 3.14
const numbers = [1, 2, 4, 8, 16]
function pies() {
// Inside this function we can "see" the variables `PI` and `numbers`
// because we are *INSIDE* the scope where they were defined
for (let index = 0; index < numbers.length; index++) {
const number = numbers[index]
const area = PI * number * number
console.log(`The area of a circle with radius ${number} is ${area}`)
// Here, we *cannot* see the variable `area` since we are *OUTSIDE* the scope
// where it was defined.


Taking the above example another step, we'll introduce the concept of closures.

NOTE: This is often an interview question.

A Closure

Closures in TypeScript are a way to create a function that has access to the variables and functions defined in the outer scope.

What does this mean? We can try a few examples.

Simple example

const variableFromOuterScope = "Wow, I'm from the outer scope"
function thisFunctionActsLikeAClosure() {
const variableFromInnerScope = 42
`I'm a closure! I have access to the variable "${variableFromOuterScope}" and the variable "${variableFromInnerScope}"`
// Have the debugger stop the program so we can look around

We'll run this program in Visual Studio, and we'll see the following output.


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  • The debugger is showing us all the variables we can see when we reach the debugger statement and our program paused.
  • The variable variableFromInnerScope is shown in the list of Local Variables.
  • The variable variableFromOuterScope is listed in the section marked closure.
  • This is because that variable was "captured" by the function when it was defined.

A more complex example.

We can create a more complex example to demonstrate that these functions do "remember" their values.

Doing work later but still having access to variables

  • Create an array of people. Each person will have a name, a birthday, and a number of milliseconds we should wait before showing their information.
  • Use TypeScript's setTimeout to do the waiting.
  • Since setTimeout calls a function later, this will help prove that the function is really "remembering" its values.

type Person = {
name: string
birthDate: string
delayMilliseconds: number
const people: Person[] = [
name: 'Alan Turing',
birthDate: 'June 23, 1912',
delayMilliseconds: 1100,
name: 'Ada Lovelace',
birthDate: 'December 10, 1815',
delayMilliseconds: 1500,
name: 'Grace Hopper',
birthDate: 'December 9, 1906',
delayMilliseconds: 2000,
name: 'Donald Knuth',
birthDate: 'January 10, 1938',
delayMilliseconds: 2500,


Then we will create a method that accepts a person variable and prints out details about them.

function printPersonInfo(person: Person) {
console.log(`${} was born on ${person.birthDate}`)

Create a loop and use setTimeout

  • Create a loop that will call the printPersonInfo method for each person in the array.
  • Call this method from inside a call to setTimeout.
  • setTimeout creates a timer that will call the supplied function at a later time.
  • Put a debugger statement inside the function so we can see what is happening.

people.forEach(function (person: Person) {
// Inside here we have access to the `person` variable. The `person` variable is
// recreated each time through the forEach loop. Since it is an argument to the
// function, it is a new creation of that variable.
function timeoutCallback() {
// We also have access to the `person` variable here. The function timeoutCallback
// will remember all the variables that existed that it could see. This includes
// the `person` variable from the forEach loop.
// When the function executes **later** (via the `setTimeout`) the
// fact that this is a "closure" (remembers variables from the outer scope) ensures
// that each time we call printPersonInfo, the variable `person` will be correct.
setTimeout(timeoutCallback, person.delayMilliseconds)

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  • TWO closures defined.
  • One created by the function(person) defined inside the forEach loop.
  • Second closure comes from inside the function timeoutCallback
  • Has access to the person variable created by the forEach loop.
  • When code calls printPersonInfo for the first time, person is Alan Turing
  • The second time, the closure captures person being equal to Ada Lovelace
  • Then Grace Hopper
  • Finally Donald Knuth.

right fit

Complex, powerful, and simple

  • Closures are when a function grabs hold of the definition of the variables it can see when defined.

  • We'll be using closures, but we won't be calling it out as a special technique.

[fit] This youtube video also gives a good overview of closures.


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