TypeScript any Type

In this tutorial, you will learn about the TypeScript any type and how to use it properly in your code.

Introduction to TypeScript any type

Sometimes, you may need to store a value in a variable. But you don’t know its type at the time of writing the program. And the unknown value may come from a third party API or user input.

In this case, you want to opt-out of the type checking and allow the value to pass through the compile-time check.

To do so, you use the any type. The any type allows you to assign a value of any type to a variable:

// json may come from a third-party API
const json = `{"latitude": 10.11, "longitude":12.12}`;

// parse JSON to find location
const currentLocation = JSON.parse(json);
console.log(currentLocation);

Output:

{ latitude: 10.11, longitude: 12.12 }

In this example, the currentLocation variable is assigned to an object returned by the JSON.parse() function.

However, when you use the currentLocation to access object properties, TypeScript also won’t carry any check:

console.log(currentLocation.x);

Output:

undefined

The TypeScript compiler doesn’t complain or issue any error.

The any type provides you with a way to work with existing JavaScript codebase. It allows you to gradually opt-in and opt-out of type checking during compilation. Therefore, you can use the any type for migrating a JavaScript project over to TypeScript.

TypeScript any: implicit typing

If you declare a variable without specifying a type, TypeScript assumes that you use the any type. This feature is called type inference. Basically, TypeScript guesses the type of the variable. For example:

let result;

In this example, TypeScript infers the type for you. This practice is called implicit typing.

Note that to disable implicit typing to the any type, you change the noImplicitAny option in the tsconfig.json file to true. You’ll learn more about the tsconfig.json in the later tutorial.

TypeScript any vs. object

If you declare a variable with the object type, you can also assign it any value.

However, you cannot call a method on it even the method actually exists. For example:

let result: any;
result = 10.123;
console.log(result.toFixed());
result.willExist(); //

In this example, the TypeScript compiler doesn’t issue any warning even the willExist() method doesn’t exist at compile time because the willExist() method might available at runtime.

However, if you change the type of the result variable to object, the TypeScript compiler will issue an error:

let result: object;
result = 10.123;
result.toFixed();

Error:

error TS2339: Property 'toFixed' does not exist on type 'object'.

Conclusion

1. The TypeScript any type allows you to store a value of any type. It instructs the compiler to skip type checking.
2. Use the any type to store a value that you don’t actually know its type at the compile-time or when you migrate a JavaScript project over to a TypeScript project.