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TypeScript Language Best Practices

Overview

Avoid using any type
Handle null & undefined Safely
Avoid Excessive Annotation
Utilize Intersection Types
Use Enums
Using Tuples for Fixed-length Arrays
Use ReadOnly and ReadOnlyArray Types
Use the Polymorphic This
Favor Type-guards over Type Assertion
Make Switch Statements Exhaustive
Use Utility Types
Use Generics
Use Conditional Types
Use Decorators

Avoid using `any` type

Pretty straightforward - any effectively disables type checking 🙂

Handle `null` & `undefined` Safely

use union type to specify if variables could be null and/or undefined

let e: string | undefined | null;

undefined type is sometimes inferred (such as for optional parameters)

function echo(param?: string) { // <- type will be `string | undefined`
    return param;
}

use non-strict equality check to test for null and undefined

function echo(param?: string): string {
    if (param != null) return param;
    return '';
}

Avoid Excessive Annotation

should try to utilize the TypeScript compiler's ability to infer types

const arr: string[] = ['a', 'b'];

arr.forEach((str: string) => { // <- could annotate callback parameter
    // do something with str...
});

arr.forEach((str) => { // <- still get full type-safety without annotation
    // do something with str...
});

Utilize Intersection Types

allow us to create new types by combining existing ones
- helps us avid type proliferation
- instead of wide range of similar types, can use small set of base types and intersections

interface Product {
    sku: string;
    description: string;
}

interface Shippable {
    destination: string;
    weight: string;
}

type ShippableProduct = Product & Shippable; // <- name is intersection of names of types

let myProduct: ShippableProduct = {
    sku: 'abc',
    description: 'a product',
    destination: 'somewhere',
    weight: '1kg',
};

NOTE: conflicting sub-types will result in never

interface Product {
    sku: string;
    description: string;
}

interface Shippable {
    destination: string;
    weight: string;
}

interface Returnable {
    returnId: string;
    weight: number; // <- this conflicts with `weight: string` from `Shippable`!
}

type ShippableReturnableProduct = Product & Shippable & Returnable; // <- TypeScript will let us create this union without any warnings

let product1: ShippableReturnableProduct = {
    sku: '123abc',
    description: '',
    destination: '',
    returnId: 'abc123',
    weight: '1kg', // <- will throw "Error: type 'string' is not assignable to type 'never'."
}

Use Enums

represent series of named values
uniquely, they survive compilation process and remain in compiled JavaScript

// Numeric enum with auto-initializing zero-based indices
enum Days {
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday,
}

// Numeric enum with explicitly assigned indices
enum Days {
    Monday = 0,
    Tuesday = 1,
    Wednesday = 2,
    Thursday = 3,
    Friday = 4,
    Saturday = 5,
    Sunday = 6,
}

// String enum
enum Days {
    Monday = 'monday',
    Tuesday = 'tuesday',
    Wednesday = 'wednesday',
    Thursday = 'thursday',
    Friday = 'friday',
    Saturday = 'saturday',
    Sunday = 'sunday',
}

Enums similar in some ways to union types and can sometimes be used in similar contexts

type Strs = 'a' | 'b' | 'c';

enum Str {
    A = 'a',
    B = 'b',
    C = 'c',
}

// Example 1
type Strs = 'a' | 'b' | 'c';

function checkString(str: Strs) {
    switch (str) {
        case 'a':
            // etc
    }
}

// Example 2
enum Str {
    A = 'a',
    B = 'b',
    C = 'c',
}

function checkString(str: Str) {
    switch (str) {
        case Str.A:
            // etc
    }
}

Using Tuples for Fixed-length Arrays

Tuples are fixed-length arrays, where each element has specific type

type Coords = [number, number];

type CoordsAndName = [
    number,
    number,
    string,
];

// Can label elements in tuple to better communicate meaning of each element
type Coords = [
    latitude: number,
    longitude: number,
];

Use `readonly` and `ReadonlyArray` Types

Since class properties/objects/arrays are mutable by default, can use readonly and ReadonlyArray types to specify whether certain things should be immutable.

Class properties should be readonly if they do not need to change after initialization

class Todo {
    constructor(
        public title: string,
        public completed: boolean,
        public readonly id: string
    ) {
        this.id = crypto.randomUUID();
    }

    public updateCompleted(status: boolean) {
        this.completed = status;
    }
}

can use readonly modifier (or ReadonlyArray generic type) to make arrays immutable

const immutable: readonly number[] = [1, 2, 3];

const immutable: ReadonlyArray<number> = [1, 2, 3];

Use the Polymorphic `this`

Fluent API

chain methods together
popularized by jQuery
see:
- Fluent interface
- Fluent API: Practice and Theory

Polymorphic this

this object can refer to different things in different contexts
don't need to do anything special to make this polymorphic

class Calculator {
    private result: number = 0;

    public add(num: number): this {
        this.result += num;
        return this;
    }

    public subtract(num: number): this {
        this.result -= num;
        return this;
    }

    public multiply(num: number): this {
        this.result *= num;
        return this;
    }

    public divide(num: number): this {
        this.result /= num;
        return this;
    }

    public reportResult(num: number): number {
        return this.result;
    }
}

Favor Type-guards over Type Assertion

type assertion tells TypeScript to treat value as if it were a specific type

class SpecificConfig {
    // ...
}

const config = state.someConfig as SpecificConfig;

type-guards offer way to protect against bugs that could be introduced by type assertions

class SpecificConfig {
    // ...
}

function isSpecificType(config: any): config is SpecificConfig {
    return config instanceof SpecificConfig;
}

Make Switch Statements Exhaustive

by "exhaustive", we mean "check every possible value of variable being switched on"
can utilize default case to handle scenarios that can/should never happen

enum Themes {
    regular = 'regular',
    dark = 'dark',
    highContrast = 'highcontrast',
};

const unhandled = (_: never): never => _;

function switchTheme(theme: Themes) {
    switch (theme) {
        case Themes.regular:
            // handle regular theme
            break;
        case Themes.dark:
            // handle dark theme
            break;
        default:
            /**
             *~ This will throw a compiler error that "Argument of type 'Themes' is
             *~ not assignable to parameter of type 'never'.
             *~ Additionally in your IDE, the type hint will specifically draw attention
             *~ to the unused property from the `Themes` enum, in this case:
             *~ `(parameter) theme: Themes.highContrast`
             */
            return unhandled(theme);
    }
}

Use Utility Types

such as Partial, Required, Record, Pick, Omit, etc.
- for a full list, see the doc page on Utility Types

Use Generics

types that take other types as parameters
- for more info, see the doc page on Generics

Use Conditional Types

basically ternary statements for types
- for more info, see the doc page on Conditional Types

type NewType = TestType extends ReferenceType ? OutputTypeA : OutputTypeB;
/**
 *~ If `TestType` is assignable to `ReferenceType`
 *~   evaluates as `OutputTypeA`
 *~ Else
 *~   evaluates as `OutputTypeB`
 */

/*------------- Practical example -------------*/
/* Before refactor */
type Log =
    | { type: 'INFO'; message: string; }
    | { type: 'ERROR'; message: string; error: Error; };

function logIt(log: Log): void {
    switch (log.type) {
        case 'INFO':
            console.log(log.message);
            break;
        case 'ERROR':
            console.log(log.message, log.error.stack);
    }
}

logIt({ type: 'INFO', message: 'I love lamp' });

/* After refactor */
type Log =
    | { type: 'INFO'; message: string; }
    | { type: 'ERROR'; message: string; error: Error; };

type LogType = Log['type'];
type ExtractNonTypeParams<L, T> = L extends { type: T } ? Omit<L, 'type'> : never;

function logIt<T extends LogType>(
    logType: T,
    args: ExtractNonTypeParams<Log, T>
): void {
    switch (logType) {
        case 'INFO':
            console.log(args.message);
            break;
        case 'ERROR':
            console.log(args.message, (args as ExtractNonTypeParams<Log, "ERROR">).error.stack);
    }
}

Use Decorators

allow us to modify behavior of class without changing class itself
TypeScript <5 used different spec (stage 2)
- requires enabling with experimentalDecorators option
TypeScript >5 aligns with ECMA spec (stage 3)
references:
- Stage 2 decorators (TS<5)
- Stage 3 decorators (TS>5)

function instrument<T extends { new (..args: any[]): object }>(target: T) {
    const originalConstructor = target.prototype.constructor;

    for (const key of Object.getOwnPropertyNames(originalConstructor.prototype)) {
        const descriptor = Object.getOwnPropertyDescriptor(originalConstructor.prototype, key);

        if (descriptor != null && typeof descriptor.value === 'function') {
            const originalFunction = descriptor.value;

            descriptor.value = function(...args: any[]) {
                console.log(`Calling method '${key}' with arguments: ${args.join(', ')}`);
                const result = originalFunction.apply(this, args);

                if (result != null) {
                    console.log(`Method '${key}' returned: ${result}`);
                }

                return result;
            };

            Object.defineProperty(originalConstructor, key, descriptor);
        }

    }
}

@instrument
class Todo {
    // ...
}

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TypeScript Language Best Practices

Overview

Avoid using `any` type

Handle `null` & `undefined` Safely

Avoid Excessive Annotation

Utilize Intersection Types

Use Enums

Using Tuples for Fixed-length Arrays

Use `readonly` and `ReadonlyArray` Types

Use the Polymorphic `this`

Favor Type-guards over Type Assertion

Make Switch Statements Exhaustive

Use Utility Types

Use Generics

Use Conditional Types

Use Decorators

TypeScript Language Best Practices

Overview

Avoid using any type

Handle null & undefined Safely

Avoid Excessive Annotation

Utilize Intersection Types

Use Enums

Using Tuples for Fixed-length Arrays

Use readonly and ReadonlyArray Types

Use the Polymorphic this

Favor Type-guards over Type Assertion

Make Switch Statements Exhaustive

Use Utility Types

Use Generics

Use Conditional Types

Use Decorators

Avoid using `any` type

Handle `null` & `undefined` Safely

Use `readonly` and `ReadonlyArray` Types

Use the Polymorphic `this`