— name: handsontable-validator-dev description: Use when creating or modifying a Handsontable cell validator – async callback-based validation functions that determine if cell values are valid — # Handsontable Validator Development ## Function signature Validators use the **callback pattern** and must always invoke the callback: “`js function myValidator(value, callback) { // `this` is bound to the cell's cellProperties object if (this.allowEmpty && (value === null || value === void 0 || value === '')) { return callback(true); } callback(isValid(value)); // true = valid, false = invalid } “` ## Key rules – **Always call the callback.** Forgetting to call `callback` hangs the validation pipeline. – **Context-aware.** `this` is the cell's `cellProperties` object. Use `this.allowEmpty`, `this.instance` (the Handsontable instance), and other cell meta properties. – **Single responsibility.** Validators only determine validity. Never modify data, DOM, or cell state. – **Async-compatible.** The callback can be called after async operations (API calls, timeouts). The editor enters WAITING state until the callback resolves. ## File structure “` src/validators/{validatorName}/ {validatorName}.js # Validator function index.js # Re-exports “` Registry: `src/validators/registry.js`. ## Registration “`js import { registerValidator } from '../../validators/registry'; registerValidator('myValidator', myValidator); “` ## How validators are triggered – Automatically by the editor system during `finishEditing()`. – Programmatically via `validateCells()`, `validateRows()`, or `validateColumns()` API methods. – Invalid cells receive the `htInvalid` CSS class (applied by the renderer pipeline, not the validator). ## Reference implementations – `src/validators/numericValidator/numericValidator.js` — Numeric validation with `allowEmpty` support. – `src/validators/dateValidator/dateValidator.js` — Date format validation. – `src/validators/autocompleteValidator/autocompleteValidator.js` — Validates against a list of allowed values. ## Common mistakes – Forgetting to call `callback`, which silently blocks editing and validation. – Not respecting `this.allowEmpty` for empty values. – Modifying data or DOM inside a validator. – Using arrow functions for the validator body (breaks `this` binding to cellProperties).