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<title>5、构造函数有__proto__属性，实例对象没有prototype 属性，原型对象有__proto__属性</title>
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<script>
function fn() {
}
console.log(fn.prototype); // {constructor: ƒ}
// {constructor: ƒ, __defineGetter__: ƒ, __defineSetter__: ƒ, hasOwnProperty: ƒ, __lookupGetter__: ƒ, …}
console.log(fn.prototype.__proto__); //
console.log(fn.__proto__); // ƒ () { [native code] }
f1 = new fn()
console.log(f1.__proto__ === fn.prototype); // true
console.log(f1.prototype); // undefined
</script>