打字稿：检查对象键和值是否适合当前的类属性

Question

I need to check if object keys and values fits to current class properties.
I have class instance and the changes in separate object then I call Object.assign to modify properties. Is there any way how to check if the object key and values are valid for certain class?

class MyClass {
  icon = 'home'
  size = 12
  color = 'blue'
}

var instance = new MyClass()

var changeProperties: MyClass = { size: 10 }
// throws Property 'icon' is missing in type { size: number; }

Object.assign(instance, changeProperties)

Now got errors for not defined properties (property icon is missing).

I have tried var changeProperties: { [string: keyof MyClass]: any } = { size: true } with no success.

Note: I can't change the class itself (eg. make class properties optional).

Answer 1

I found the answer in lib.es6.d.ts

/**
 * Make all properties in T optional
 */
type Partial<T> = {
    [P in keyof T]?: T[P];
};

So the answer is:

class MyClass {
  icon = 'home'
  size = 12
  color = 'blue'
}

var instance = new MyClass()

var changeProperties: Partial<MyClass> = { size: 10 }

Object.assign(instance, changeProperties)

Answer 2

We can always tell TS that we know more, ie

// instead of this
var changeProperties: MyClass = { size: 10 }
// we can assure TS that we know more than it..
var changeProperties: MyClass = { size: 10 } as MyClass

That is assertion - a statement for TS - instructing it that the passed object is really what it should be

Answer 3

Not sure if I understand correctly, but maybe something like that

class MyClass {
   icon = 'home'
   size = 12
   color = 'blue'
}

//just copied from your code, I don't know the purpose of it
var instance = new MyClass()

var changeProperties: MyClass = new MyClass();
var properties = { size: 10, hello: 'test'};

for(const property of Object.getOwnPropertyNames(properties)) {

   if(instance.hasOwnProperty(property) && typeof instance[property] === typeof properties[property]) {

       instance[property] = properties[property];
   }
}

Maybe there is a better solution but that's how I solved something similar.

if you look at instance after the loop you will see that hello has been ignored and is not assigned. the same happens if you change size in properties to a string. instance.size will then still be 12.

hope it helps.

See resources:

• Object.getPropertyNames
• typeof

It will only work wit IE9+

PS I only tested and used it with simple types like string, numbers etc.