Checking whether toString has been assigned

This question already has an answer here:

How do I check if an object has a property in JavaScript? 22 answers

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像obj.hasOwnProperty('toString')

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The result of obj.hasOwnProperty('toString') depends on the way how your toString implementation is created. I did some simple test:

function A() {
}

A.prototype.toString = function(){
   console.log("A.toString");
};

function B() {
   this.toString = function(){
      console.log("B.toString");
   }
}

var t = [new A(), new B(), new A()];

for(var i=0; i<t.length; i++) {
    var hasOwnToString = t[i].hasOwnProperty("toString");
    console.log(hasOwnToString);

    if(hasOwnToString) {
       t[i].toString();
    }
}

Result:

false
true
B.toString
false

For A style it's false , but for B style it's true

---

You can check to see if the toString function is the same as Object (the object all objects descend from)

function hasUserPrototype(obj) {
    return Object.getPrototypeOf(obj).toString !== Object.prototype.toString;
}

See Check if an object has a user defined prototype?

function Foo() 
{
}

// toString override added to prototype of Foo class
Foo.prototype.toString = function()
{
    return "[object Foo]";
}

var f = new Foo();

function Boo() 
{
}

var g = new Boo();

function hasUserPrototype(obj) {
    return Object.getPrototypeOf(obj).toString !== Object.prototype.toString;
}

document.write(hasUserPrototype(f)); // true
document.write(hasUserPrototype(g)); // false

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