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function deepCopy(obj) {
 return JSON.parse(JSON.stringify(obj)); // serialize -> deserialize -> unique clone
}

const original = { 
   a: undefined, 
   b: { c: undefined }, 
   d: () => undefined 
};

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function deepCopy(obj) {
 return JSON.parse(JSON.stringify(obj)); // serialize -> deserialize -> unique clone
}

const animals = {
  zebra: {
    food: ['leaves', 'bark'],
    name: 'zebra'
  },
  panda: {

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export type Immutable<T> = T extends (infer R)[]
  ? ImmutableArray<R>
  : T extends Function
  ? T
  : T extends object
  ? ImmutableObject<T>
  : T;

interface ImmutableArray<T> extends ReadonlyArray<Immutable<T>> {}

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const obj = {
  a: 1,
  b: 2,
  c: {
    d: 3
  }
};

obj.newField = 5;   // You can assign new field

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// npm install tslint-immutable --save-dev
// tslint.json
{
  "extends": ["tslint-immutable"],
  "rules": {

    // Recommended built-in rules
    "no-var-keyword": true,
    "no-parameter-reassignment": true,
    "typedef": [true, "call-signature"],

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/*
   INCORRECT CHANGES (deep mutation)
*/

const animals = {
  zebra: {
    food: ['leaves', 'bark'],
    name: 'zebra'
  },
  panda: {

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const animals = {
  zebra: {
    food: ['leaves', 'bark'],
    name: 'zebra'
  },
  panda: {
    food: [],
    name: 'panda'
  }
};

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@NgModule({
  imports: [
    BrowserModule,
    NgxsModule.forRoot([ AppState ], { developmentMode: !env.production })
  ],
  ...
})
export class AppModule { }

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<b>default</b>: {{ todos$ | async | json }} <br>
<b>immutable reverse</b>: {{ immutableTodos$ | async | json }} <br>
<b>mutable reverse</b>: {{ mutableTodos$ | async | json }} <br>

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@State<number[]>({
  name: 'todos',
  defaults: [1, 2, 3]
})
export class AppState {

  @Selector() public static mutableTodos(state: number[]): number[] {
    return state.reverse();
  }