What impact does assigning a variable with the type of 'any' have on the type of the variable being assigned?

Question

What impact does assigning a variable with the type of 'any' have on the type of the variable being assigned?

Consider the code snippet below:

let x: string = "hello world"
let y: number = 23
let z: any = 23

console.log(typeof x) // "string"
console.log(typeof y) // "number"
console.log(typeof z) // "number"

x = y // Fails!
x = z // Works!
console.log(typeof x) // "number"

x = y // Fails!
x = "Hallo Welt" // Works!
console.log(typeof x) // "string"

What allows changing the type of a non-'any' variable to 'any'? Shouldn't it be
```
x: any = y: string | number | ...
```
, instead of
```
x: string | number | ... = y: any
```
, unless the 'any' variable is inferred to match a specific type?
Why can a string be assigned after converting the type to number, but not a number?
Is there a way to prevent a variable from ever changing its type?

javascript typescript types any

Answer 1

Answer №1

As per the TypeScript Documentation:

Avoid using any as a type unless you're converting a JavaScript project to TypeScript. The compiler essentially treats any as "please disable type checking for this item". It's like placing an @ts-ignore comment around every use of the variable. This can be useful when initially moving from a JavaScript project to TypeScript by setting the type to any for parts that haven't been converted yet, but in a fully TypeScript project, you end up turning off type checking for sections of your program where it's used.

If you're unsure about the type you want to accept, or if you need to accept anything without manipulation, then consider using unknown.

let a: string = "hello world"
let b: number = 23
let c: unknown = 23

console.log(typeof a) // "string"
console.log(typeof b) // "number"
console.log(typeof c) // "number"

a = b // Fails!
a = c // Fails!
a = "Hallo Welt" //Works!

Answer 2

As per the TypeScript Documentation:

Avoid using any as a type unless you're converting a JavaScript project to TypeScript. The compiler essentially treats any as "please disable type checking for this item". It's like placing an @ts-ignore comment around every use of the variable. This can be useful when initially moving from a JavaScript project to TypeScript by setting the type to any for parts that haven't been converted yet, but in a fully TypeScript project, you end up turning off type checking for sections of your program where it's used.

If you're unsure about the type you want to accept, or if you need to accept anything without manipulation, then consider using unknown.

let a: string = "hello world"
let b: number = 23
let c: unknown = 23

console.log(typeof a) // "string"
console.log(typeof b) // "number"
console.log(typeof c) // "number"

a = b // Fails!
a = c // Fails!
a = "Hallo Welt" //Works!

What impact does assigning a variable with the type of 'any' have on the type of the variable being assigned?

Answer №1

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