some progress

node_modules/mongoose/lib/error/validation.js

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+/*!
+ * Module requirements
+ */
+
+'use strict';
+
+const MongooseError = require('./mongooseError');
+const getConstructorName = require('../helpers/getConstructorName');
+const util = require('util');
+const combinePathErrors = require('../helpers/error/combinePathErrors');
+
+class ValidationError extends MongooseError {
+  /**
+   * Document Validation Error
+   *
+   * @api private
+   * @param {Document} [instance]
+   * @inherits MongooseError
+   */
+  constructor(instance) {
+    let _message;
+    if (getConstructorName(instance) === 'model') {
+      _message = instance.constructor.modelName + ' validation failed';
+    } else {
+      _message = 'Validation failed';
+    }
+
+    super(_message);
+
+    this.errors = {};
+    this._message = _message;
+
+    if (instance) {
+      instance.$errors = this.errors;
+    }
+  }
+
+  /**
+   * Console.log helper
+   */
+  toString() {
+    return this.name + ': ' + combinePathErrors(this);
+  }
+
+  /**
+   * inspect helper
+   * @api private
+   */
+  inspect() {
+    return Object.assign(new Error(this.message), this);
+  }
+
+  /**
+  * add message
+  * @param {String} path
+  * @param {String|Error} error
+  * @api private
+  */
+  addError(path, error) {
+    if (error instanceof ValidationError) {
+      const { errors } = error;
+      for (const errorPath of Object.keys(errors)) {
+        this.addError(`${path}.${errorPath}`, errors[errorPath]);
+      }
+
+      return;
+    }
+
+    this.errors[path] = error;
+    this.message = this._message + ': ' + combinePathErrors(this);
+  }
+}
+
+
+if (util.inspect.custom) {
+  // Avoid Node deprecation warning DEP0079
+  ValidationError.prototype[util.inspect.custom] = ValidationError.prototype.inspect;
+}
+
+/**
+ * Helper for JSON.stringify
+ * Ensure `name` and `message` show up in toJSON output re: gh-9847
+ * @api private
+ */
+Object.defineProperty(ValidationError.prototype, 'toJSON', {
+  enumerable: false,
+  writable: false,
+  configurable: true,
+  value: function() {
+    return Object.assign({}, this, { name: this.name, message: this.message });
+  }
+});
+
+
+Object.defineProperty(ValidationError.prototype, 'name', {
+  value: 'ValidationError'
+});
+
+/*!
+ * Module exports
+ */
+
+module.exports = ValidationError;