preValidateProperty + documentation

README.md

@@ -172,6 +172,42 @@ them:
   importNextSchema();
 ```
 
+### Pre-Property Validation Hook
+If some processing of properties is required prior to validation a function may be passed via the options parameter of the validate function. For example, say you needed to perform type coercion for some properties:
+
+```const coercionHook = function (instance, property, schema, options, ctx) {
+  var value = instance[property];
+
+  // Skip nulls and undefineds
+  if (value === null || typeof value == 'undefined') {
+    return;
+  }
+
+  // If the schema declares a type and the property fails type validation.
+  if (schema.type && this.attributes.type.call(this, instance, schema, options, ctx.makeChild(schema, property))) {
+    var types = (schema.type instanceof Array) ? schema.type : [schema.type];
+    var coerced = undefined;
+
+    // Go through the declared types until we find something that we can
+    // coerce the value into.
+    for (var i = 0; typeof coerced == 'undefined' && i < types.length; i++) {
+      // If we support coercion to this type
+      if (lib.coercions[types[i]]) {
+        // ...attempt it.
+        coerced = lib.coercions[types[i]](value);
+      }
+    }
+    // If we got a successful coercion we modify the property of the instance.
+    if (typeof coerced != 'undefined') {
+      instance[property] = coerced;
+    }
+  }
+}.bind(validator)
+
+// And now, to actually perform validation with the coercion hook!
+v.validate(instance, schema, { preValidateProperty: coercionHook });
+```
+
 ## Tests
 Uses [JSON Schema Test Suite](https://github.com/json-schema/JSON-Schema-Test-Suite) as well as our own tests.
 You'll need to update and init the git submodules:

lib/attribute.js

@@ -187,6 +187,10 @@ validators.properties = function validateProperties (instance, schema, options,
   var result = new ValidatorResult(instance, schema, options, ctx);
   var properties = schema.properties || {};
   for (var property in properties) {
+    if (typeof options.preValidateProperty == 'function') {
+      options.preValidateProperty(instance, property, properties[property], options, ctx);
+    }
+
     var prop = (instance || undefined) && instance[property];
     var res = this.validateSchema(prop, properties[property], options, ctx.makeChild(properties[property], property));
     if(res.instance !== result.instance[property]) result.instance[property] = res.instance;
@@ -214,6 +218,11 @@ function testAdditionalProperty (instance, schema, options, ctx, property, resul
     });
   } else {
     var additionalProperties = schema.additionalProperties || {};
+
+    if (typeof options.preValidateProperty == 'function') {
+      options.preValidateProperty(instance, property, additionalProperties, options, ctx);
+    }
+
     var res = this.validateSchema(instance[property], additionalProperties, options, ctx.makeChild(additionalProperties, property));
     if(res.instance !== result.instance[property]) result.instance[property] = res.instance;
     result.importErrors(res);
@@ -242,6 +251,11 @@ validators.patternProperties = function validatePatternProperties (instance, sch
         continue;
       }
       test = false;
+
+      if (typeof options.preValidateProperty == 'function') {
+        options.preValidateProperty(instance, property, patternProperties[pattern], options, ctx);
+      }
+
       var res = this.validateSchema(instance[property], patternProperties[pattern], options, ctx.makeChild(patternProperties[pattern], property));
       if(res.instance !== result.instance[property]) result.instance[property] = res.instance;
       result.importErrors(res);