如何清除打字稿中的类型化数组并保留其类型？

Question

Now in a class, I declared an array with type: CustomType, like below

class Example {
  public exampleArray: CustomType[];
  public clearArray() {
    this.exampleArray = [];
  }
}

As you can see the clearArray assign an UNDEFINED type of an empty array, which seems lost the type information.

How I can clear the array but preserving its declared type?

Answer 1

The type information is determined by the type annotation on the field ( exampleArray: CustomType[] ). At runtime Javascript arrays are untyped anyway. The compiler will allow an empty array ( [] ) to be assigned to anything as this is considered safe, since there are no objects inside, it can be an array of CustomType . The field type will then prevent you form pushing to the array objects of any other type:

class CustomType { x: number}
class Example {
    public exampleArray: CustomType[];
    public clearArray() {
        this.exampleArray = [];
        this.exampleArray.push(new CustomType())
        this.exampleArray.push({}) // error not a `CustomType`
    }
}

Note

If this had been a variable with no type annotation, the variable would have been inferred to any[] , which can lead to problems (types are not checked on either when assigning an array literal or when you would push to the array):

let noAnnotationArray = [] //  any[]

In this case, adding a type annotation is still the best way to go. Type assertions (as suggested in other answers) can lead to uncaught errors if later someone adds an item to the array:

let withAnnotation:CustomType[] = [{}] //  error
let withAssertion = <CustomType[]>[{}] // no error even though we assign {}

Answer 2

This post seems to be out of date. There is a new way to do this.

this.array = [] as CustomType[];

Answer 3

You can always clear the list items by changing the length property to 0

eg: this.exampleArray.length = 0

Setting length to 0 clears all the elements in the array without altering the reference of the array.

Answer 4

There can be 3 ways of emptying an array

1. setting its length = 0

   myArr.length = 0;

2. using splice method Array

   myArr.splice(0,myArr.length);

3. Pop() each element of array

   while(myArr.length){ myArr.pop(); }

Answer 5

你可以试试

this.exampleArray = <CustomType[]>[];

Answer 6

即使通过方法作为参数传递，也要清理数组：

myArray.length = 0;