unique

Asynchronously checks that the value is unique in the backend.

Signature

NguardValidators.Async.unique(config: AsyncValidatorConfig): AsyncValidatorFn

AsyncValidatorConfig

Field	Type	Default	Description
endpoint	string	—	URL to check against
method	'GET' | 'POST' | 'PUT'	'GET'	GET sends the value as a query param; POST/PUT in the body
paramName	string	'value'	Key carrying the value
debounceTime	number	300	Debounce (ms) before the request
resolve	(response: HttpResponse | HttpErrorResponse) => boolean | null	status-code rule	Your own verdict from the full response — true = valid, false = invalid, null = undecided (valid)

Default rule (no resolve)

Status code only — the library never assumes a response body shape:

Response	Meaning	Result
2xx	the value exists (taken)	invalid { unique: true }
404	not found (free)	valid
anything else (5xx, network)	undecided	valid (don't block)

Reactive forms

import { FormControl } from '@angular/forms';
import { NguardValidators } from 'ng-nguard';

// In a component field (an injection context — the factory calls inject(HttpClient))
username = new FormControl('', {
    asyncValidators: [NguardValidators.Async.unique({ endpoint: '/api/users/check-username' })],
});

Template-driven forms

<input ngModel name="username" [nguardUnique]="{ endpoint: '/api/users/check-username' }" />

Custom rule

Most backends are not REST-compliant and won't signal availability by status code — so define your own rule from the whole response (status, headers, or a body however it is nested):

import { HttpResponse } from '@angular/common/http';

NguardValidators.Async.unique({
    endpoint: '/api/users/check-username', // always returns 200 with { data: { taken: boolean } }
    resolve: (res) => (res instanceof HttpResponse ? (res.body as { data: { taken: boolean } }).data.taken === false : null),
});

Error key

{ unique: true }

Notes

• Must be created in an injection context (component field/constructor); the directive handles this for you.
• Debounced; Angular cancels the in-flight request on the next change. control.pending is true while it runs.
• Empty values pass without a request. Transport/server failures resolve to undecided (valid) so they don't block submission.

See also

• uniqueExcept — ignore the current record on edit forms
• exists — the opposite check