TypeScript is a programming language that adds types to JavaScript. It allows us to write JavaScript with a set of tools called a type system that can spot potential bugs in, document the shapes of, and help refactor our code.

Types

Because TypeScript knows what _types_ our objects are, it also knows what _shapes_ our objects adhere to. An object's shape describes, among other things, what  properties and methods it does or doesn't contain.

The built-in types in JavaScript each have known properties and methods that always exist. All `string`s, for example, are known to have a `.length` property and `.toLowerCase()` method.

```ts
"OH".length; // 2
"MY".toLowerCase(); // "my"
```

TypeScript's `tsc` command will let you know if your code tries to access properties and methods that don't exist:

```ts
"MY".toLowercase();
// Property 'toLowercase' does not exist on type '"MY"'.
// Did you mean 'toLowerCase'?
```
Through this knowledge of type shapes, TypeScript helps us quickly locate bugs in our code. 

Let's try it!

Type Shapes

JavaScript allows us to assign any value to any variable. That makes it very flexible to use, which can be good for getting started quickly in coding. In practice, variables that are assigned values of multiple types throughout a program can be very confusing or lead to bugs.

One of the first things we’ll discover with TypeScript is that when we declare a variable with an initial value, the variable can never be reassigned a value of a different data type. This is an example of _type inference_: everywhere in our program, TypeScript expects the data type of the variable to match the type of the value initially assigned to it at declaration.

TypeScript recognizes JavaScript's built-in "primitive" data types:
* boolean
* number
* null
* string
* undefined

If we try to reassign a variable to a value of a different type, TypeScript will surface an error. 

```typescript
let order = 'first';

order = 1;
```
Running the TypeScript code above will result in the following error being surfaced in the terminal: `Type '1' is not assignable to type 'string'`.

TypeScript’s type system is telling us that `order` is expected to be the primitive type `string`, but we’re trying to assign it a value of type `number`. That’s not allowed: variables can only be assigned the types expected by the type system.

We can fix this complaint by changing the new value to be the expected `string` type:

```typescript
let order = 'first';

order = '1';
```
Let's practice!

Type Inferences

There are some places where TypeScript will not try to infer what type something is—generally when a variable is declared without being assigned an initial value. In situations where it isn't able to infer a type, TypeScript will consider a variable to be of type `any`. 

Variables of type `any` can be assigned to _any_ value and TypeScript won't give an error if they're reassigned to a different type later on.

```typescript
let onOrOff;

onOrOff = 1;
onOrOff = false;
```

In the code above, we declared the variable `onOrOff` without an initial value. TypeScript considers it to be of type `any`, and, therefore, doesn't produce an error when we change the variable's assignment from a `number` value to a `boolean` value. 


So now that we know _why_ TypeScript exists, let's talk about how we use it:
+ First, we write TypeScript code in files with the extension **.ts**. 
+ Next, we run our code through the TypeScript [transpiler](https://en.wikipedia.org/wiki/Source-to-source_compiler). The transpiler will check that the code adheres to TypeScript's standards, and it will display errors when it does not. 
+ If the TypeScript code can be converted into working JavaScript, the transpiler will output a JavaScript version of the file (**.js**).

TypeScript code is a _superset_ of JavaScript code—it has all the features of traditional JavaScript but adds some new features. 

Given this TypeScript code as input:

```typescript
let firstName = 'Anders';
```

The TypeScript transpiler would output this JavaScript code:

```javascript
let firstName = 'Anders';
```

That’s right, they’re the same! TypeScript code generally looks roughly the same as the equivalent JavaScript. 

What we'll learn throughout this lesson and course is the real joy of using TypeScript—that it helps us understand more about our code and, particularly, discover bugs.

What is TypeScript?

In some situations, we'd like to declare a variable without an initial value while still ensuring that it will only ever be assigned values of a certain type. If left as `any`, TypeScript won't be able to protect us from accidentally assigning a variable to an incorrect type that could break our code. 

We can tell TypeScript what type something is or will be by using a _type annotation_. 

Variables can have _type annotations_ (also known as type declarations) added just after their names.  We provide a type annotation by appending a variable with a colon (`:`) and the type (e.g., `number`, `string`, or `any`). 

```typescript
let mustBeAString : string;
mustBeAString = 'Catdog';

mustBeAString = 1337;
// Error: Type 'number' is not assignable to type 'string'
```
In the code above, we explicitly declare `mustBeAString` to be of type `string` without assigning it an initial value. This enables us to assign it the value `'Catdog'` without complaint, but when we later attempt to assign it a numerical value, TypeScript will give us an error message telling us that a `number` is being improperly assigned to a variable of type `string`.

We're becoming familiar with how useful type annotations can be! Some developers may find that type annotations make the code wordy or hard for others to understand, however, they get automatically removed when compiled to JavaScript. 

Variable Type Annotations

Invented in 1995, JavaScript was designed as a small scripting language for simple web pages in browsers. It wasn't until 1999 that JavaScript was capable of supporting the kinds of dynamic web pages we see today, and using JavaScript that way wasn't common practice until 2005.

To serve its original use-case, JavaScript was designed to be very flexible and easy to use for small applications. These features make JavaScript a great first language to learn, but they also make it less-than-ideal for building larger-scale applications with hundreds or even thousands of files. Stricter programming languages will inform the developer when they change one area of code in a way that will break other areas. JavaScript will not, which often leads to unexpected behavior at runtime. 

To address these shortcomings, Microsoft developed TypeScript and released publicly in 2012 to blend the flexibility of JavaScript with the advantages of a stricter language. 

TypeScript is a programming language that adds _types_ to JavaScript. It allows us to write JavaScript with a set of tools called a _type system_ that can spot potential bugs in, clarify the structure of, and help refactor our code. In addition, TypeScript added newer JavaScript language features, such as arrow functions and classes, years before they were added to JavaScript officially.

Today, TypeScript is one of the [most loved languages](https://insights.stackoverflow.com/survey/2019#most-loved-dreaded-and-wanted) in the world, from open source projects such as Angular and Webpack to large-scale development at Amazon and Google. We use it at Codecademy!


Timeline of fun internet technology from the 90s to now.

From JavaScript to TypeScript

All right! You’ve taken some first steps into the wonderful world of type safety with TypeScript. 💪

To recap, you’ve learned:
* _TypeScript_  is a superset of JavaScript that adds types.
* The _type system_ refers to TypeScript’s understanding of how your code is meant to function: mainly what data types should be stored under your variables.
* TypeScript expects the data type of the variable to match the type of the value initially assigned to it at its declaration—this is known as _type inference_.
* An object’s _shape_ describes, among other things, what properties and methods it does or doesn’t contain. TypeScript knows not only what type something is but also what shape that type has. 
* When it isn’t able to infer a type, TypeScript will consider a variable to be of type `any`.
* _Type annotations_ are little pieces of code that indicate what type a variable is meant to be.

```typescript
let youAreAwesome: boolean;
youAreAwesome = true;
```
