请解释类型断言

Question

// Type Assertion
let value: any = "Asim Hussain";
let len: number = (<string>value).length;

I don't really understand this. Please explain!

Answer 1

let value: any = "Asim Hussain";

This lets the variable value to be in any kind of type. It can be an integer, string, array, object.. Well, any thing!

let value: any = "Asim Hussain"; // OK

value = 0; // OK
value = []; // OK

---

let len: number = (<string>value).length;

This forces the variable len to be in the number type, which only allows number s such as 0, 0.233, etc.

let value: any = "Asim Hussain;" 
let len: number = (<string>value).length;

len = "213"; // Error
len = 213; // OK
len = value; // Error
len = value.length; // OK

The .length function returns a number, which is what the varable desires.

Answer 2

let value: any = "Asim Hussain";
The variable value can contain anything (type any )

let len: number = (<string>value).length;
The variable len can only contain number (type number )
The value (type any ) is cast into a type string .
(<string>value) == value.toString()
Tha variable len is set to the length of this string .

Answer 3

You're telling typescript that you know more than typescript.

Example 1:

let value: any = "Asim Hussain"
const t = value.map(s => s)

Result: Typescript won't give me any errors.

Example 2

let value: any = "Asim Hussain"
const t = (<string>value).map(s => s)

Result: Typescript lets me know 'Property map does not exist on string'

Example 3

let value: any = "Asim Hussain"
const len = value.length

Result: The type of len would be any aswell.

Example 3 I tell Typescript that I'm using length of a string

let value: any = "Asim Hussain"
const len = (<string>value).length

Results: Typescript knows that len is a number since String.prototype.length returns a number.