TypeScript Booleanish inputs

const toBoolean = (context: any, flags: string[]) => {
  for (const flag of flags) {

    const propValue = context[flag];
    const propType = typeof propValue;

    if ('boolean' === propType) {
      context[flag] = propValue;
    }

    else if ('string' == propType) {
      if (propValue === 'false' || propValue === 'no') {
        context[flag] = false;
      } else {
        context[flag] = true;
      }
    }

    else {
      context[flag] = !!propValue;
    }
  }
};

interface ICommon {
  booleanish: string | boolean;
}

class MyClass {
  flag: ICommon['booleanish'];

  constructor(flag: ICommon['booleanish']) {
    this.flag = flag;
    toBoolean(this, ['flag']);
  }
}

const obj = new MyClass('no');

console.log(obj.flag);