TypeScript any Type

let item: any;

item = "A string"; // first, item variable is a string
item = 100;        // now, its a number
item = false;      // now, its a boolean

console.log(item); 

// Output: false

Declare any Type Variable

To declare a variable with the any type, you simply use the any keyword in the type annotation. For example,

let variableName: any;

Declaring a variable as any tells the compiler to disable all type-checking for that variable.

Using any With Function

You can use any to declare function arguments.

// Function that accepts any type of user detail
function processUserDetail(userDetail: any) {
    console.log(userDetail);
}

// Calls function with a string
processUserDetail("Jack"); 

// Calls function with a number
processUserDetail(27); 

// Calls function with a boolean
processUserDetail(true); 

Output

Jack
27
true

Here, we've defined a function processUserDetail() that accepts a parameter userDetail of type any.

We've passed three types of arguments in the function:

• A string "Jack"
• A number 27
• A boolean true

Example: Using any With an Object

let employeeDetails: any = {
    name: "Carol",
    age: 35
}

// Adding a new property "country" to employeeDetails
employeeDetails.country = "UK";

console.log(employeeDetails);

In this example, we've used any with the employeeDetails object. This allows employeeDetails to accept new property country without TypeScript errors.

Now, let's see what happens if we don't use any in the employeeDetails object and instead use the defined object structure.

let employeeDetails: { name: string; age: number} = {
    name: "John",
    age: 30,
};

// Attempt to add the country property 
employeeDetails.country = "UK";

console.log(employeeDetails);

Here, we've attempted to add a new property country to employeeDetails but it throws an error.

Without using any, TypeScript ensures that you can only add or modify properties that are mentioned in the definition.

Disadvantages of Using any

Using any in TypeScript offers flexibility, however, it also introduces several disadvantages. Some of them are:

1. Loss of Type Safety

Using any disables TypeScript's type checking for that variable. For example,

let data: any = "Hello";
console.log(data.toUpperCase());  // Works fine, outputs: "HELLO"
data = 123;
console.log(data.toUpperCase());  // Runtime error: data.toUpperCase is not a function

• Initially, data is a string, so toUpperCase() works.
• After changing data to a number, trying toUpperCase() results in a runtime error because numbers don't have this method.
• The any type prevents TypeScript from catching this at compile time, leading to a runtime error.

2. Makes Refactoring Risky

Without type checks, refactoring code can become hazardous. Consider an object details declared with any.

let details: any = { name: "Alice", age: 30 };

If we refactor "name" to "firstName", and then attempt to access the original name property, TypeScript won't flag this as an error at compile time:

console.log(details.name);  // No TypeScript error, yet it will be undefined at runtime.

3. Decrease code understandability and maintainability

Overuse of any can make the codebase difficult to understand and maintain. For example,

let item: any = { id: 1, title: "New Item" };

Here, it is difficult to understand what "item" can be or should be over time.