Handsontable Validator Development

Signature

function myValidator(value, callback) {
  // `this` is bound to the cell's cellProperties object
  if (this.allowEmpty && (value === null || value === undefined || value === '')) {
    return callback(true);
  }
  callback(isValid(value)); // true = valid, false = invalid
}

Rules

• Call the callback on every path; a missed call hangs the validation pipeline and silently blocks editing.
• this is cellProperties: use this.allowEmpty, this.instance, other cell meta. Use a function body, not an arrow function (arrows break the this binding).
• Validators only determine validity; data, DOM, and cell state stay untouched.
• The callback may run after async work; the editor stays in WAITING state until it resolves.

Files

src/validators/{validatorName}/
  {validatorName}.ts    # Validator function
  index.ts              # Re-exports

Registry: src/validators/registry.ts.

import { registerValidator } from '../../validators/registry';
registerValidator('myValidator', myValidator);

Triggers

• The editor system during finishEditing().
• validateCells(), validateRows(), validateColumns().
• Invalid cells get the htInvalid class from the renderer pipeline, not the validator.

Reference implementations

• src/validators/numericValidator/numericValidator.ts - allowEmpty support.
• src/validators/dateValidator/dateValidator.ts
• src/validators/autocompleteValidator/autocompleteValidator.ts - list of allowed values.

Correcting cell values inside a validator

When a validator writes a corrected value with setDataAtCell, pass a source string ending in 'Validator':

this.instance.setDataAtCell(row, col, correctedValue, 'myCustomValidator');

validateChanges() in core.js uses the suffix to track the correction. Without it, the correction is lost when the batch includes columns with async source callbacks (e.g. async autocomplete with strict: true) that resolve later.

Common mistakes

• Reading a meta flag that an editor forces in prepare(). DropdownEditor.prepare() writes strict = true onto the one cell it prepares, only while selected. A validator reading this.strict sees the column's own strict: false on every cell never clicked (paste, setDataAtCell(), validateCells(), any updateSettings() that rebuilds cell meta), which made a strict: false dropdown strict only in clicked cells (DEV-2911). For fixed cell-type behavior, pass it to the shared logic explicitly: dropdownValidator calls validateAgainstSource(this, value, callback, true). Test by writing to a cell that was never selected; selecting it first hides the bug.