object.assign does not copy functions

Question

I have a common typescript class with some functions.

When I do this.selected = Object.assign({} as AssignedTestType, newTestType);

the selected instance does not own that functions which are in the type AssignedTestType.

How can I make a 'deep' copy with object.assign? or any alternative method?

Answer 1

object.assign does not copy functions

That's untrue

let x = {a: y => y * 2}
let z = Object.assign({}, x)
console.log(z.a(5)); // 10

the selected instance does not own that functions which are in the type AssignedTestType.

Now this part is true. Object.assign will only do a shallow copy and will only enumerate own props

Answer 2

If the properties you're looking to copy are in the prototype of newTestType, they will not be copied, since Object.assign only copies an object instance's own properties.

The alternatives are to either instantiate whatever constructor you used to create newTestType, and assign the resulting instance to this.selected, or to create an empty object and set its prototype to newTestType. In either case, Object.assign is the wrong tool to use here.

Answer 3

How much effort you spend on cloning an object will depend on what you want to do.

First port of call should be .clone or .cloneDeep.

Also, based on the discussion you might be talking about copying references to functions on the prototype. In that case the solution might be as simple as:

class MyClass { foo() {} }
class MyOtherClass extends MyClass {}
console.log(new MyOtherClass().__proto__.foo); // foo from MyClass

If you want to see a hacky clone function, this is a (untested) start (but they can get complex quite fast depending on your requirements):

function clone(o) {
    if(!isObject(o)) {
        throw 'o must be a a non-function object';
    }    
    return (function inner(a, b = {}) {
        Object.keys(a).forEach(k => {
          isObject(a[k])? b[k] = inner(a[k]) : b[k] = a[k];
        });    
        return b;
    }(o));
}

function isObject(o) {
    return o !== null && typeof o === 'object'
}

var a = {
    foo: 'foo',
    bar: () => { console.log('bar'); },
    bat: {
        baz: () => { console.log('baz'); }
    }
};

console.log(clone(a));

Answer 4

Fast clone complex object:

Code

public static deepClone<T>(obj: any) {
if ( obj === null || obj === undefined) {
  return obj;
} else if ( Array.isArray(obj)) {
  const array: T[] = [];
  obj.forEach( item => array.push( this.deepClone<typeof item>(item) ));
  return array as T[];
} else {
  const c = Object.assign({} as T, obj);
  const fields: string[] = Object.getOwnPropertyNames(obj);
  fields.forEach( f => {
    const field = obj[f];
    if ( typeof field === 'object' ) {
      c[f] = this.deepClone<typeof field>(field);
    }
  });
  return Object.assign({}, obj);
}

}

Example:

const x = {myProp: 'befor change value', c: {a: 1 , b: 2}, d: [1, 2, 3], func: () => {console.log('befor change function'); }};
const y = Object.assign(x);
const z = Utils.deepClone<any>(x);
console.log(x, y, z);
x.c.a = 12;
x.myProp = 'after change value';
x.func = () => console.log('after change function');
console.log(x, y, z);