TypeScript is a popular programming language that has gained immense popularity among developers due to its ability to enforce strong typing and detect errors at compile time. However, like any programming language, TypeScript has its quirks and nuances that can cause headaches for even the most experienced developers. In this article, we will take a look at some of the top TypeScript mistakes that you need to avoid, with code snippets for each mistake.

Mistake #1: Not using strictNullChecks

One of the biggest advantages of TypeScript is that it can detect null or undefined values at compile time. However, this feature is not enabled by default, and you need to turn it on explicitly by setting strictNullChecks to true in your tsconfig.json file.

Consider the following code snippet:

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let foo: string;
foo = null; // Compilation Error

Without strictNullChecks, the above code will compile without any errors. However, if you turn on strictNullChecks, you will get a compilation error stating that “Type ’null’ is not assignable to type ‘string’”.

Mistake #2: Using the any type too often

TypeScript provides the any type as a way to bypass type checking. While it can be useful in some cases, using it too often can defeat the purpose of using TypeScript in the first place. When you use the any type, you lose the benefits of strong typing and type safety.

Consider the following code snippet:

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function add(x: any, y: any): any {
  return x + y;
}

In the above code, we have used the any type for all the function parameters and return type. This means that TypeScript will not perform any type checking for these values. As a result, we may end up with runtime errors that could have been caught at compile-time if we had used more specific types.

Mistake #3: Using non-null assertions excessively

TypeScript provides a non-null assertion operator (!) that allows you to tell the compiler that a value is not null or undefined, even if it may appear so. While this operator can be useful in some cases, using it excessively can lead to unexpected errors at runtime.

Consider the following code snippet:

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let foo: string | null = null;
let bar = foo!.length; // Runtime Error

In the above code, we have used the non-null assertion operator to access the length property of the foo variable, even though it is assigned a null value. This will result in a runtime error, as we are trying to access a property of a null value.

Mistake #4: Not using interface or type aliases

TypeScript provides interface and type aliases as a way to define custom types. These types can be used to make the code more readable and maintainable, as well as provide additional type safety.

Consider the following code snippet:

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function printPerson(person: { name: string; age: number }) {
  console.log(`Name: ${person.name}, Age: ${person.age}`);
}

In the above code, we have defined a function that takes an object with two properties, name, and age. While this code may work fine for small projects, it can quickly become unwieldy for larger projects with many different types. By using an interface or type alias, we can make the code more readable and maintainable:

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interface Person {
  name: string;
  age: number;
}

function printPerson(person: Person) {
  console.log(`Name: ${person.name}, Age: ${person.age}`);
}

Mistake #5: Not using the correct function signature

Another common TypeScript mistake is not using the correct function signature. TypeScript uses function signatures to define the types of function parameters and return values. If you do not use the correct function signature, you may end up with runtime errors that could have been caught at compile time.

Consider the following code snippet:

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function add(x: number, y: number): string {
  return x + y;
}

In the above code, we have defined a function that takes two number parameters and returns a string. This function signature is incorrect, as we are returning a string instead of a number. This will result in a runtime error if the function is called, as we are trying to return a string instead of a number.

Mistake #6: Not using generics

TypeScript provides generics as a way to write reusable code that can work with multiple types. Using generics can make the code more flexible and easier to maintain.

Consider the following code snippet:

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function identity(arg: any): any {
  return arg;
}

In the above code, we have defined a function that takes any argument and returns it. While this code may work fine for simple cases, it can quickly become problematic if we need to ensure type safety. By using generics, we can make the code more reusable and type-safe:

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function identity<T>(arg: T): T {
  return arg;
}

In the above code, we have defined a generic function that takes an argument of type T and returns it. The use of the type parameter T makes the function more flexible and type-safe.

Mistake #7: Not using enums

TypeScript provides enums as a way to define a set of named constants. Using enums can make the code more readable and maintainable, as well as provide additional type safety.

Consider the following code snippet:

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const STATUS_ACTIVE = "Active";
const STATUS_INACTIVE = "Inactive";

function getUserStatus(status: string) {
  switch (status) {
    case STATUS_ACTIVE:
      return "Active";
    case STATUS_INACTIVE:
      return "Inactive";
    default:
      return "Unknown";
  }
}

In the above code, we have defined two string constants to represent user statuses. While this code may work fine for small projects, it can quickly become unwieldy for larger projects with many different statuses. By using an enum, we can make the code more readable and maintainable:

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enum UserStatus {
  Active = "Active",
  Inactive = "Inactive",
  Unknown = "Unknown",
}

function getUserStatus(status: UserStatus) {
  switch (status) {
    case UserStatus.Active:
      return "Active";
    case UserStatus.Inactive:
      return "Inactive";
    default:
      return "Unknown";
  }
}

Conclusion

TypeScript is a powerful programming language that provides strong typing and type safety. However, like any programming language, it has its quirks and nuances that can cause headaches for developers. In this article, we have looked at some of the top TypeScript mistakes that you need to avoid, with code snippets for each mistake. By avoiding these mistakes and following best practices, you can write cleaner, more readable, and more maintainable TypeScript code.