I don’t understand why I become in my Angular project the TypeScript error Type boolean is not assignable to type never when I try to set a value to the testObject.

export interface TestInterface {
  text: string;
  number: number;
  bool1: boolean;
  bool2: boolean;
}

export class DamagesComponent implements OnInit {
  testObject: TestInterface = {
    text: 'test String',
    number: 22,
    bool1: false,
    bool2: true
  }

  ngOnInit() {
    this.toogleVal('bool1')
  }

  toogleVal(key: keyof TestInterface): void {
    if (key in this.testObject) {
      if (typeof this.testObject[key] === 'boolean') {
        // here the error
        this.testObject[key] = true;
      }
    }
  }
}

>Solution :

When you are accessing the property dynamically, you need to specify in the interface that the key will be a string and the return type will be some data type, the error will go away by modifying the interface to.

export interface TestInterface {
  text: string;
  number: number;
  bool1: boolean;
  bool2: boolean;
  // this needs to be at the bottom for it to accept the default datatypes of the properties set already!
  [key: string]: string | boolean | number;
  // [key: string]: any; // or with any data type which is less preferred!
}

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