feat: 添加实现 instanceof 笔记

docs/javaScript/实现instanceof.md

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+# 实现 instanceof
+
+`instanceof` 用于判断构造函数的 `prototype` 属性是否出现在一个对象的原型链中。
+
+```js
+function Car(make, model, year) {
+  this.make = make;
+  this.model = model;
+  this.year = year;
+}
+var auto = new Car('Honda', 'Accord', 1998);
+console.log(auto instanceof Car); // true
+console.log(auto instanceof Object); // true
+```
+## instanceof
+
+```js
+/**
+ * instanceof
+ *
+ * @param {Object} obj 需要判断的对象.
+ * @param {Function} ctor 构造函数.
+ * @returns {Boolean} 返回判断结果.
+ */
+function instanceof(obj, ctor) {
+  var proto = obj.__proto__;
+  var prototype = ctor.prototype;
+  while (true) {
+    if (proto == null) return false;
+    if (proto == prototype) return true;
+    proto = proto.__proto__;
+  }
+}
+```
+
+## 参考
+
+- [instanceof](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof)
+- [instanceof原理](https://juejin.im/post/5b7f64be51882542c83476f0)