Generics page
Learn about what Generics are, why they are useful, and how to create a linked list using Generics in TypeScript.
Overview
In this part, we will:
- Understand the purpose and basics of generic functions
- Understand how to make generic classes
- Understand how to make recursive generic classes
- Create a TreeNode recursive generic class
Basic Generics
Generics are a way of writing abstract code that allows the determination of types to be handled when the code is used. Generics let us reuse code for different types and improve maintainability. Lets see how with a small example.
Consider a function that wraps a value in an object:
function wrapAsValue(value) {
return {value: value};
}
Ideally, you'd want to use this function to wrap all sorts of values:
let fourObj = wrapAsValue(4); //-> {value: 4}
let hiObj = wrapAsValue("hi"); //-> {value: "hi"}
And you might want to pass those objects to other functions:
function getDollars(obj: {value: number}){
return "$"+obj.value.toFixed(2)
}
function getMessage(obj: {value: string}) {
return obj.value + " world";
}
getDollars(fourObj); //-> "$4.00"
getMessage(hiObj); //-> "hi world"
But watch out! The following will not error until runtime
because strings do not have a toFixed()
method.
getDollars(hiObj);
You don't see a compile time error because hiObj
object looks like {value: any}
to TypeScript.
Getting a compile time error can be solved in a variety of inelegant ways:
- Way 1 - Define the type of the variables:
let fourObj: {value: number} = wrapAsValue(4); let hiObj: {value: string} = wrapAsValue("hi");
- Way 2 - Write multiple functions:
function wrapStringAsValue(value: string) { return {value: value}; } function wrapNumberAsValue(value: number) { return {value: value}; }
- Way 3 - Overload
wrapAsValue
signatures:function wrapAsValue(value: string): {value: string}; function wrapAsValue(value: number): {value: number}; function wrapAsValue(value: any) { return {value: value}; }
With generics, this problem can be solved more simply:
function wrapAsValue<MyType>(value: MyType): {value: MyType} {
return {value: value};
}
let fourObj = wrapAsValue<number>(4);
let hiObj = wrapAsValue("hi");
function getDollars(obj: {value: number}){
return "$"+obj.value.toFixed(2)
}
function getMessage(obj: {value: string}) {
return obj.value + " world";
}
getDollars(fourObj);
getMessage(hiObj);
getDollars(hiObj);
The <MyType>
part of the wrapAsValue
definition is the Generics
part. This <MyType>
allows us to capture the type the user provides, so that we can use that information later. In this case, we are using it to specify that the
return type is an object with a MyType
value
property ({value: MyType}
). This allows us to traffic that type information in one side of the function and out the other.
We can call generic functions in two ways:
We can explicitly pass the type:
wrapAsValue<number>(4)
Notice that
<number>
acts as a special set of arguments. Instead of arguments passed likefunc(arg1, arg2, arg3)
, generic type arguments are passed likefunc<Type1, Type2, Type3>
.The type can be inferred:
wrapAsValue("hi")
Notice that we didn't explicitly pass the type n the angle brackets (
<>
). Instead, the compiler just looked at the value"hi"
and setMyType
tostring
.
Generic Classes
Generic classes are quite common. For example, RxJS subjects are a generic class that can publish values of a particular type:
const cardNumber = new Subject<string>();
cardNumber.next("1234")
Let's look at making a basic class to collect a list of things.
class Collection {
private list: any[] = [];
push(thing) {
this.list.push(thing);
}
}
The good - we can push any type to this list.
The bad - we can push any type to this list.
let myList = Collection();
myList.push(25);
myList.push('25');
myList
now holds an assortment of types and will be a likely source of
runtime errors.
Let's build a generic Collection
class instead.
class GenericCollection<T> {
private list: T[] = [];
pushItem( thing:T ) {
this.list.push(thing);
}
}
Now when we initialize this class we can specify a type to use.
class GenericCollection<T> {
private list: T[] = [];
pushItem(thing:T) {
this.list.push(thing);
}
}
let myListOfStrings = new GenericCollection<string>();
myListOfStrings.pushItem('booop');
myListOfStrings.pushItem(5);
//error Argument type of '5' is not assignable to parameter of type 'string'
let myListOfNumbers = new GenericCollection<number>();
myListOfNumbers.pushItem(5);
myListOfNumbers.pushItem('boop');
//error Argument type of '"boop"' is not assignable to parameter of type 'number'
interface Dinosaur {
name: string;
breed: string;
teeth: number;
}
let myListOfDinosaurs = new GenericCollection<Dinosaur>();
let otherDino = {
name: 'Blue',
breed: 'Velociraptor',
teeth: 100
}
myListOfDinosaurs.pushItem(otherDino);
myListOfDinosaurs.pushItem({name: 'Charlie'});
//error Argument type '{ name: string; }' is not assignable to parameter of type 'Dinosaur'.
Recursive Generic Classes
A great example of the power of generics is creating a linked list with type safety. We will create a simple linked list that supports:
- Adding values to the front of the list with
linkedList.unshift(value)
. - Removing and returning the front values with
linkedList.shift()
. - Reading the front of the list with
linkedList.head
. - Reading the end of the list with
linkedList.tail
.
We can use it with strings like:
var linkedList = new LinkedList<string>();
linkedList.unshift("a");
linkedList.unshift("b");
console.log( linkedList.shift() ) //logs "b"
console.log( linkedList.shift() ) //logs "a"
Or with numbers like:
var linkedList = new LinkedList<number>();
linkedList.unshift(100);
linkedList.unshift(200);
console.log( linkedList.head ) //logs 200
console.log( linkedList.tail ) //logs 100
The implementation looks like this:
// Define node that has a value and points to the
// next item in the list.
class LinkedListNode<T> {
value: T;
next?: LinkedListNode<T>;
constructor(val: T) {
this.value = val;
this.next = null;
}
}
class LinkedList<T> {
private _head: LinkedListNode<T>;
private _tail: LinkedListNode<T>;
// Adds to the start of the list.
unshift(value: T) {
var node = new LinkedListNode(value);
// The existing head is now next.
if(this._head) {
node.next = this._head;
}
this._head = node;
// If there wasn't a tail, this is the first node
if(!this._tail) {
this._tail = node;
}
}
// removes first
shift(){
let value: T;
// If there was a head,
// set head to whatever is after it.
if(this._head) {
value = this._head.value;
this._head = this._head.next;
}
// If there is no more head, the
// list is empty.
if(!this._head) {
this._tail = null;
}
return value;
}
get head() { return this._head.value }
get tail() {return this._tail.value }
}
Thanks to generics we're able to use the same LinkedList
class in multiple different scenarios with any type.
Exercise: TreeNode
The Problem
Update the 6-tree-node.ts
file to create a recursive TreeNode
class that can house a value
and be used to create a tree structure of left
and right
nodes.
For example, we will be able to create a TreeNode
with a root value and
comparison function as follows:
function stringComparison(v1: string, v2: string): number {
if(v1 > v2) {
return 1;
} else {
return -1;
}
};
let root = new TreeNode<string>("Jennifer", stringComparison);
Then we can add values to root
like:
root.add("Chasen");
This will add Chasen
to a left
TreeNode
of root
because
the stringComparison
will return 1
(Jennifer > Chasen
):
root.left.value //-> "Chasen"
As we add other values, they will be added to either the right or left nodes recursively:
root.add("Tom");
root.add("Matthew");
root.right.value //-> "Tom"
root.right.left.value //-> "Matthew"
Verify Your Solution
✏️ Run the following to verify your solution:
npm run 6-generics
The Solution
Click to see the solution
✏️ Update 6-tree-node.ts
to the following:
interface Comparison<T> {
(v1:T,v2: T): number;
}
class TreeNode<T> {
value: T;
compare: Comparison<T>;
left?: TreeNode<T>;
right?: TreeNode<T>;
constructor(val: T, compare: Comparison<T> ) {
this.value = val;
this.compare = compare;
}
add(val: T){
if( this.compare(this.value, val) >= 1 ) {
if(this.left == null) {
this.left = new TreeNode(val, this.compare);
} else {
this.left.add(val);
}
} else {
if(this.right == null) {
this.right = new TreeNode(val, this.compare);
} else {
this.right.add(val);
}
}
}
}
export default TreeNode;