我们如何在打字稿中定义对象类型接口？

Question

I want to define a interface with object and different types such as

export interface example {
   code: string;
   category : {
     name : string,
     reference: string,
     sequence : number
   };
}

In definition, there is no problem but after calling like

ex = {} as example;
ex.category.name ='electric; 

this does not work and below error occurs

ERROR Error: Uncaught (in promise): TypeError: Cannot set property 'name' of undefined TypeError: Cannot set property 'name' of undefined

There are some similar subjects but they are not exactly related. ( How to define object in type script interface or How can I define the types of an object variable in Typescript? )

I appreciate your assistance finding the solution.

Answer 1

Type assertions do not mean the object will definitely be of the shape you are asserting at runtime. You can assert any object is of any type, but ultimately it will fail at runtime if your runtime type does not match.

In your example, the ex object does not have a category property so it will be undefined at runtime which leads to your error.

You can initialize the category property as well in your object:

var ex = {
    category: {} // or you can initialize `name` here as well
} as example;
ex.category.name = 'electric';