class-transformer returning undefined-CodePudding

I am new to using the class-transformer. I created a simple product class and JSON string to load a Product. However, I get undefined, even though I see the output when I log the result like the transformation was successful. I removed the readonly, but it made no difference. Please help, I have spent so much time. The problem is reading the JSON from a json file does not work. When I use the JSOn as follows:

   let prodJson = { 
                   name: 'MyProduct,
                   productId: 'POOA',
                   description: 'A product'
                  }

etc. this works. If I read the JSON from a file, it doesn't.

 export class Product{  
   @Expose() 
   public readonly name: string;
   @Expose()
   public readonly productId?: string;
   @Expose()
   public readonly description?: number; 
  }

   const productJson =   {\n"  
       "     \"name\": \"Aerosol\",\n"  
       "     \"productId\": \"ARSL\",\n"  
       "     \"description\": \"Apply to living room\"\n"       
      "}"

      
      createProduct(productJson: string){
         return plainToInstance(Product, productJson);
      }

      const product = createProduct(productJson);
      console.log('Product', product); // undefined

CodePudding user response：

createProduct(productJson: string){
  return plainToInstance(Product, JSON.parse(productJson));
}

plainToInstance: This method transforms a plain javascript object to instance of specific class. The second parameter not a json string. it should be an object.