Why is any dangerous in a project?

TypeScriptJunior~3 min read

What `any` does

The any type tells the compiler: "Don't check this piece of code at all - I take responsibility for it."

That is, TS stops tracking which properties, methods, and types you use. The code compiles even if it can potentially crash at runtime.

Example:

javascript

let user: any = { name: "Alex" };

user();              // no error, even though user is not a function
user.age.toFixed();  // no error, even though age does not exist

TypeScript stays silent, but in JS this code causes a crash.

1. `any` disables type checking entirely

With any, TypeScript stops warning about problems:

javascript

let value: any = 123;
value = "hello";
value = { x: 10 };
value.nonexistentMethod(); // crashes at runtime, but TS does not complain

any turns a variable into a type-level "black box" - any call and any access inside it is considered valid.

2. Errors move from compile-time to runtime

Without any, TypeScript would catch the error in advance.

javascript

function greet(name: string) {
  console.log(name.toUpperCase());
}

greet(42); // compile-time error

With any, the compiler lets it through:

javascript

function greet(name: any) {
  console.log(name.toUpperCase()); // runtime error if name is a number
}

The code compiles, but breaks only when it runs. You lose TypeScript's main advantage - catching errors early.

3. `any` "infects" other types

any behaves like a virus: one any variable can "corrupt" an entire section of the type system.

Example:

javascript

let value: any = "hello";

let upper = value.toUpperCase(); // ok
let len: number = upper;         // ok, even though upper is a string

Because of any, TS considers any assignment valid - so type safety disappears down the whole chain.

4. Autocomplete and IDE help disappear

TypeScript can suggest methods and properties, but for any it does not know what is actually available:

javascript

let user: any;
user. // the IDE does not know what to suggest

This makes the code less predictable and slows down development.

TypeScript cannot track where and how any is used. If you rename a property or method, the IDE cannot guarantee nothing broke.

Example:

javascript

interface User { name: string; }

let u: any = { name: "Tom" };

// later
u.fullName; // the IDE will not flag that 'fullName' does not exist

any kills TypeScript's "magic" features: refactor, rename, jump-to-definition, and so on.

6. It hampers teamwork

When there is a lot of any in the code, it becomes impossible to safely know what a function returns or an object contains.

Example:

javascript

function getData(): any {
  // ...
}

Neither you nor your teammates know what getData() returns. The IDE cannot help, and no one can be sure that calling getData().user.name will not throw an error.

7. "any" breaks type compatibility

The type system stops working correctly:

javascript

function sum(a: number, b: number) {
  return a + b;
}

const result = sum(1, "2" as any); // TS lets it through, even though this is a string!

Instead of 3 you get "12". TS cannot help, because any "muted" the type check.

8. `any` often hides real design mistakes

When a type does not compile, it is sometimes easier to "silence" TS with any:

javascript

const data: any = fetchUser(); // "I'll figure it out later"

This solves the problem quickly right now, but a month later no one remembers that the type needed to be checked here. Eventually the project turns into "JS with annotations".

9. `any` breaks control over the API

If functions returning any end up in a public API, all consumers lose their typing.

Example:

javascript

// utils.ts
export function parseJSON(json: string): any {
  return JSON.parse(json);
}

Anyone who imports parseJSON loses autocomplete, hints, and safety. Better like this:

javascript

export function parseJSON<T>(json: string): T {
  return JSON.parse(json) as T;
}

10. `any` cannot be safely checked

TS does not verify that typeof or instanceof are applicable to any:

javascript

let val: any;

if (val instanceof Date) {
  // TS cannot guarantee this is even an object
}

Any condition on any is meaningless from the type system's point of view.

11. It can mask type holes

TypeScript is not always able to infer a type, and instead of an error it just "substitutes" any implicitly - if you have strict mode turned off (noImplicitAny: false).

Example:

javascript

function add(a, b) {
  return a + b;
}

Both parameters automatically become any, so the function is not typed at all. That's why you should always enable "noImplicitAny": true in tsconfig.json.

When `any` is acceptable (rarely!)

Sometimes any is genuinely appropriate:

Rapid prototyping.
A temporary import of a JS library without types.
Handling "black box" data (for example, JSON.parse without certainty about its structure).
Legacy code where you are gradually adding typing.

But even in these cases it is better to replace any with safer alternatives:

Instead of `any`	Use	Description
`any`	`unknown`	A safe variant: requires a type check
`any`	`never`	For expressions that should never exist
`any`	`Record<string, unknown>`	For dynamic objects
`any`	A generic (`<T>`)	For generic functions

Example: `unknown` is safer than `any`

javascript

let value: unknown = "hello";

value.toUpperCase(); // Error - the type must be checked first

if (typeof value === "string") {
  value.toUpperCase(); // safe
}

unknown forces you to prove the type, while any simply "closes its eyes".

Summary

Problem	What `any` does
Type checking	Disables it entirely
Autocomplete	Lost
Runtime errors	Hidden until the code runs
Spread	Infects other types
Refactoring	Breaks
Readability	Becomes unclear
Safety	Reduced to zero

In simple terms:

any is a "black hole" for typing. Convenient at the start, but deadly dangerous in long-term projects. If TypeScript is insurance, any is a hole in the parachute.

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