4 techniques for using TypeScript effectively to improve code quality
Published: 2023-04-25 06:58:31

TypeScript is a statically typed superset of JavaScript that offers many benefits such as type safety, better tooling support, and improved code readability. 

TypeScript can make your codebase more maintainable and less prone to errors if used correctly.

In this article, we will discuss 4 advanced TypeScript techniques that can help you write cleaner, more maintainable code.

### 1. Use Advanced Types:
TypeScript provides many advanced types that can make your code more expressive and concise. Some examples include:

#### Union Types
Union types allow you to define a variable that can hold values of different types. For example, the following code defines a variable that can hold either a string or a number:

`let address: string | number;
address = "Lagos"
address = 19;`

Here we define a variable called `address` which can hold values of either type string or number. 

We then assign a string value "Lagos" to the address variable. Since we have defined address as a string or number variable, this is perfectly valid.

Similarly, we assign a number value 19 to the `address` variable, which is also valid since address is defined to accept either a string or a number.

This is an example of using Union Types in TypeScript to define a variable that can hold multiple types.

Using Union Types can be helpful when you have a variable that can hold values of different types, and you want to enforce type safety by ensuring that only certain types of values are assigned to it.

#### Intersection Types:
Intersection types allow you to combine multiple types into a single type. For example, the following code defines a type that has properties from both Name and Age interfaces:

`interface Name {
  name: string;
}

interface Age {
  age: number;
}

type UserInfo = Name & Age;

const info: UserInfo = {
  name: 'james',
  age: 28,
};`

Here we define two interfaces named `Name` and `Age`, and a type named `UserInfo`. 

The `Name` interface has a single property called name of type string and the `Age` interface also has a single property called age of type number.

We then define `UserInfo` type using the `&` (intersection) operator, which means it combines the properties of both `Name` and `Age` interfaces. This creates a new type that has all the properties of both interfaces.

The info constant is of type `UserInfo`, which means it has to have both the name and age properties. In this case, info is an object that has a name property with the value of 'james' and an age property with the value of 28.


#### Conditional Types
Conditional types allow you to define types that depend on other types. For example, the following code defines a type that returns a number if T is a number, and a string otherwise

`type NumberOrString<T> = T extends number ? number : string;

const a: NumberOrString<number> = 42; // a is of type number
const b: NumberOrString<string> = 'hello'; // b is of type string`

Here, we define a generic type called `NumberOrString`. The generic type T is used to represent the input type that will be passed to `NumberOrString`.

The `NumberOrString` type is defined using a conditional type that uses the extends keyword to create a type that can either be number or string depending on the type of the generic type parameter T.

In this case, the conditional type is checking whether T extends number. If it does, then the resulting type is number. If it does not, then the resulting type is string.

#### Type Guards:
Type guards allow us to narrow down the type of a variable based on a condition. For example, the following code checks whether a variable is an array:

`function isArray(value: any): value is Array<any> {
  return Array.isArray(value);
}

const arr: any = [1, 2, 3];

if (isArray(arr)) {
  // arr is now of type Array<any>
  console.log(arr.length);
}`

Here, we define an function `isArray` that takes an argument value of type `any` and returns a boolean value indicating whether value is an array or not. 

The return type of this function is value is Array<any>, which means that if the function returns true, TypeScript should infer that the input value is an array of any type.

The implementation of the `isArray` function simply checks whether the input value is an array using the built-in Array.isArray method, and returns the result.

### 2. Use Interfaces and Types
Interfaces and types are the bread and butter of TypeScript. They allow us to define custom types and enforce type safety. 
When defining types, it is important to follow some best practices:

    Always use interfaces for public API signatures.
    Use types for private implementation details.
    Use types to define complex types or type aliases.

Here's an example of using interfaces and types:

`interface IUser {
  id: number;
  name: string;
  email: string;
}

type UserList = Array<IUser>;

function getUsers(): Promise<UserList> {
  // fetch users from the server
}`

Here, we define an interface called `IUser` with three properties: id which is a number, name which is a string, and email which is also a string. 
The interface `IUser` defines the structure of an object representing a user.

Next, we created a type alias called `UserList` which is an array of `IUser` objects. This type is used to define the expected return type of the `getUsers` function.

Finally, the `getUsers` function is defined as an asynchronous function that returns a Promise of type `UserList`.

#### 3. Use Enums:
Enums allow you to define a set of named constants. They can be used to make your code more expressive and self-documenting. Here's an example:

```
enum LogLevel {
  Debug,
  Info,
  Error,
}

function log(level: LogLevel, message: string) {
  console.log(`[${LogLevel[level]}] ${message}`);
}

log(LogLevel.Info, 'Hello, world!');
```

Here, we define an `enum` called `LogLevel`. `enum` help us to define a set of named constants.

In this case, `LogLevel` has three constants: `Debug, Info, and Error`. By default, the values of these constants start at 0 and increment by 1, but we can explicitly set the values if needed. 

In this case, Debug has a value of 0, Info has a value of 1, and Error has a value of 2.

We then define a function called `log` that takes two parameters: `level`, which is of type `LogLevel`, and `message`, which is a string. 
The function logs the message to the console with a timestamp and the level of the log message.

The log function is called with `LogLevel`.Info as the level and the string 'Hello, world!' as the message. This will result in the following output in the console:

`[Info] Hello, world!`

### 4. Use Decorators
Decorators are a feature of TypeScript that allows us to modify classes and class members at runtime. They can be used to add functionality to our classes, such as logging or caching. Here's an example:

```
function log(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  const originalMethod = descriptor.value;

  descriptor.value = function (...args: any[]) {
    console.log(`Calling ${propertyKey} with args
```

Here, we define a function called `log` that takes three parameters: `target`, `propertyKey`, and `descriptor`. These parameters are part of the JavaScript/TypeScript language feature called `decorators`, which can be used to modify or enhance the behavior of a class or its members.

When this `log` function is invoked, it first creates a variable called `originalMethod` that holds the original value of the function that is being decorated. 

In other words, the `log` function is intercepting the call to the original function, so it can modify its behavior or log additional information.

Next, the descriptor.value is replaced with a new function that logs a message before invoking the original method. This new function takes an arbitrary number of arguments using the spread syntax (...args: any[]), and logs a message to the console using the console.log method.

In conclusion, this article explored various advanced techniques for using TypeScript effectively to improve code readability and maintainability. 

We covered 4 techniques, including the use of advanced types such as Union Types, Intersection Types, Conditional Types, and Type Guards, as well as the use of Interfaces, Types, Enums, and Decorators.

By leveraging these advanced TypeScript features, you can write cleaner and more expressive code, enforce type safety, and make your codebase more maintainable and less prone to errors. 

Additionally, by using these techniques consistently throughout your codebase, you can improve code consistency and make it easier for developers to understand and maintain your code.