Check out example codes for "unique validator angular reactive form". It will help you in understanding the concepts better.

Code Example 1

// How to check for unique validity in MEAN stack apps
// Backend : Node + Express + MongoDB
// DB-backend communication : mongoose + mongoose_unique_validator
// Angular script to show validations errors in UI
// You have to match angular's formControlNames with mongoose's model fields

// Init
fg: FormGroup
constructor(private $http: HttpClient) {}

// your http request
this.$http.post('//my_post_url', data).subscribe(
  (e) => {
  	console.log('All is well')
  },
  (error) => {
    console.error('we got errors')
    Object.keys(error.errors)
      .map((errorKey) => errors[errorKey].path)
      .forEach((path) => {
      	// path: name of mongoose schema field where unique error occured
        this.fg.get(path).setErrors({ notUnique: true })
      })
  }
)

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