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This lesson will teach you all about TypeScript's array types. 

Arrays

So far we've looked at arrays with all elements of the same type. But, as we know, JavaScript arrays are flexible and can have elements of different types. With TypeScript, we can also define arrays with a fixed sequence of types: 

```ts
let ourTuple: [string, number, string, boolean] = ['Is', 7 , 'our favorite number?' , false]; 
```

In TypeScript, when an array is typed with elements of specific types, it's called a _tuple_. The tuple above (`ourTuple`) contains the elements: `'Is'`, `7` , `'our favorite number?'` , `false` and the tuple has a type of `[string, number, string, boolean]`. Tuple types specify both the lengths and the orders of compatible tuples, and will cause an error if either of these conditions are not met:
```typescript 
let numbersTuple: [number, number, number] = [1,2,3,4]; // Type Error! numbersTuple should only have three elements.
let mixedTuple: [number, string, boolean] = ['hi', 3, true] // Type Error! The first elements should be a number, the second a string, and the third a boolean. 
```
\
As far as JavaScript is concerned, tuples act just like arrays. They both have `.length` properties. We can access (or change) the elements of both using `[index]`.  But despite their similarities, tuples and arrays do not have compatible types within TypeScript. Specifically, we can't assign an array to a tuple variable, even when the elements are of the  correct type: 
```typescript 
let tup: [string, string] = ['hi', 'bye'];
let arr: string[] = ['there','there'];
tup = ['there', 'there']; // No Errors.
tup = arr; // Type Error! An array cannot be assigned to a tuple.
```
\
Now let's practice using tuples and tuple types!

Tuples 

Beautiful! Array types may seem like a monster topic, but now you've got it tamed. You learned:
* The type annotations for arrays.
* What tuples are, and how to do their type annotations.
* How type inference works with arrays and tuples.
* How to use the rest and spread operators with TypeScript. 

Arrays types are a crucial component of TypeScript, so great job!


Review

Like the finest wines and cheeses, TypeScript's tuples pair beautifully with JavaScript's [spread syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax). This is most useful for function calls that use lots of arguments, like this: 

```typescript 
function gpsNavigate(startLatitudeDegrees:number, startLatitudeMinutes:number, startNorthOrSouth:string, startLongitudeDegrees: number, startLongitudeMinutes: number, startEastOrWest:string, endLatitudeDegrees:number, endLatitudeMinutes:number , endNorthOrSouth:string, endLongitudeDegrees: number, endLongitudeMinutes: number,  endEastOrWest:string) {
  /* navigation subroutine here */
}
```
The function call `gpsNavigate(40, 43.2, 'N', 73, 59.8, 'W', 25, 0, 'N', 71, 0, 'W')` calculates a route from the Codecademy offices in New York City (40 degrees  43.2 minutes north, 73 degrees 59.8 minutes west) to selected coordinates in the Bermuda Triangle. We all agree that this function call is awkward to read. 

Instead, we can use tuple variables that represent the starting and ending coordinates:

```typescript 
let codecademyCoordinates: [number, number, string, number, number, string] = [40, 43.2, 'N', 73, 59.8, 'W'];
let bermudaTCoordinates: [number, number, string, number, number, string] = [25, 0 , 'N' , 71, 0, 'W'];
```

These tuple type annotations guarantee that the types of the elements will be valid function parameters for `gpsNavigate()`.

Now, we use JavaScript's spread syntax to write a very readable function call:
```typescript
gpsNavigate(...codecademyCoordinates, ...bermudaTCoordinates);
// And by the way, this makes the return trip really convenient to compute too:
gpsNavigate(...bermudaTCoordinates, ...codecademyCoordinates);
// If there is a return trip . . . 
```
\
Now, you practice:

Spread Syntax

The TypeScript type annotation for array types is fairly straightforward: we put `[]` after the element type. 
In this code, `names` is an Array that can only contain `string`s:

```typescript 
let names: string[] = ['Danny', 'Samantha'];
```

An alternative method is to use the `Array<T>` syntax, where `T` stands for the type.

```typescript 
let names: Array<string> = ['Danny', 'Samantha'];
```

In the code above, the type, `T`, is `string`. We won't use `Array<T>` syntax in this lesson, but it's nice to be familiar with it.

As we may expect, we get a type error if we try to assign an array of `number`s to a `string[]` variable:

```typescript 
let names: string[] = [1,2,3]; // Type Error!
```

That was just like an assignment error with primitive types. TypeScript arrays, however, can also throw errors when elements of the wrong type are added:

```typescript 
let names: string[] = ['Damien'];
names.push(666) // Type Error!
```

Let's practice some array type annotations!

Array Type Annotations

Our TypeScript journey has now led us to a new destination: Array-bia. Typing arrays is a little bit different than working with primitive types. This is because arrays usually contain many pieces of data. Keeping track of the array’s type means keeping track of every element's type. For example:

```typescript 
let firstArray = [1, 2, 3, 4];
let secondArray =  [5, '6', [7]];
```

We can see that `firstArray` has elements that are all `number` types. On the other hand, `secondArray` has elements of varying types: `number`, `string`, and `Array`. Both are examples of valid JavaScript arrays. As we'll soon explore, TypeScript makes it very easy to keep track of element types in arrays, such as those above.


For now, though, let's make life difficult by pretending that TypeScript didn't have special ways of typing arrays. How hard would it be to enforce type consistency?


Introduction

TypeScript can infer variable types from initial values and return statements. Even still, we may not know exactly what type inference to expect when dealing with arrays. For example: 
```typescript 
let examAnswers= [true, false, false];
```
What is the type of `examAnswers`? It seems it could equally well be `boolean[]` or `[boolean, boolean, boolean]`. In reality, it is always the first of these, since this is the less restrictive type. This enables us to expand the array:

```ts
examAnswers[3] = true; // No type error.
```

Since tuples have fixed lengths, we wouldn't be able to add additional boolean elements to a tuple:

```ts
let tupleOfExamAnswers: [boolean, boolean, boolean] = [true, false, false];
tupleOfExamAnswers[3] = true; // Type error! The tuple only has 3 elements.
```

We also get the same kind of type inference when we use the `.concat()` method:
```typescript 
let tup: [number, number, number] = [1,2,3];
let concatResult = tup.concat([4,5,6]); // concatResult has the value [1,2,3,4,5,6].
```
In the code above, TypeScript infers the variable `concatResult` as an array of numbers, not a tuple.
 
The takeaway here is that type inference returns arrays. When we want tuples, we need to use explicit type annotations. 


Array Type Inference

Assigning types to [rest parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters)  is similar to assigning types to arrays. Here's a rest parameter example without types:

```typescript 
function smush(firstString, ...otherStrings){
  let output = firstString;
  for(let i = 0; i < otherStrings.length; i++){
    output = output.concat(otherStrings[i]);
  }
  return output;
}
```

This function concatenates all of its arguments. For example, calling: `smush('hi ', 'there')` returns the value `'hi there'`." The _rest parameter_ `otherStrings` lets the function work with any number of parameters greater than zero: 
```typescript
smush('a','h','h','H','H','H','!','!'); // Returns: 'ahhHHH!!'.
``` 
\
The function works well, but it is not type safe. We don't want a misguided coder to make a mistake like `smush(1,2,3)`, when that would cause an error. TypeScript to the rescue! Type annotations for a rest parameter are identical to type annotations for arrays. The function with a correctly typed rest parameter is then:
```typescript
function smush(firstString, ...otherStrings: string[]){
  /*rest of function*/
}
```
With this change, TypeScript will treat `otherStrings` as an array of strings. This means that 
`smush(1,2,3)` will result in a type error because `[2,3]` is not an array of strings.

Now, it's your turn to write a rest parameter type!

Rest Parameters

So far we've looked at `string[]` arrays, but we could also have arrays that only contain `number`s (`number[]`) or `boolean`s  (`boolean[]`). In fact, we can make arrays of any type whatsoever. We can also declare multidimensional arrays: arrays of arrays (of some type). 

```typescript
let arr: string[][] = [['str1', 'str2'], ['more', 'strings']];
```

Think of the `string[][]` above as short for `(string[])[]`, that is, an array where every element has type `string[]`. We'll explore more complex array types in later lessons.

The empty array (`[]`) is compatible with any array type: 
```typescript 
let names: string[] = []; // No type errors.
let numbers: number[] = []; // No type errors.
names.push('Isabella');  
numbers.push(30);
```

Practice time!