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node_modules/express-validator/docs/feature-custom-validators-sanitizers.md

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+---
+id: custom-validators-sanitizers
+title: Custom validators/sanitizers
+---
+
+Although express-validator offers plenty of handy validators and sanitizers through its underlying
+dependency [validator.js](https://github.com/chriso/validator.js), it doesn't always suffices when
+building your application.
+
+For these cases, you may consider writing a custom validator or a custom sanitizer.
+
+## Custom validator
+A custom validator may be implemented by using the chain method [`.custom()`](api-validation-chain.md#customvalidator).
+It takes a validator function.
+
+Custom validators may return Promises to indicate an async validation (which will be awaited upon),
+or `throw` any value/reject a promise to [use a custom error message](feature-error-messages.md#custom-validator-level).
+
+### Example: checking if e-mail is in use
+```js
+const { body } = require('express-validator');
+
+app.post('/user', body('email').custom(value => {
+  return User.findUserByEmail(value).then(user => {
+    if (user) {
+      return Promise.reject('E-mail already in use');
+    }
+  });
+}), (req, res) => {
+  // Handle the request
+});
+```
+
+### Example: checking if password confirmation matches password
+```js
+const { body } = require('express-validator');
+
+app.post('/user', body('passwordConfirmation').custom((value, { req }) => {
+  if (value !== req.body.password) {
+    throw new Error('Password confirmation does not match password');
+  }
+  
+  // Indicates the success of this synchronous custom validator
+  return true;
+}), (req, res) => {
+  // Handle the request
+});
+```
+
+## Custom sanitizers
+Custom sanitizers can be implemented by using the method `.customSanitizer()`, no matter if
+the [validation chain one](api-validation-chain.md#customsanitizersanitizer) or
+the [sanitization chain one](api-sanitization-chain.md#customsanitizersanitizer).  
+Just like with the validators, you specify the sanitizer function, which _must_ be synchronous at the
+moment.
+
+### Example: converting to MongoDB's ObjectID
+```js
+const { sanitizeParam } = require('express-validator');
+
+app.post('/object/:id', sanitizeParam('id').customSanitizer(value => {
+  return ObjectId(value);
+}), (req, res) => {
+  // Handle the request
+});
+```