JavaScript instanceof operator returning false

Question

var name = new String("green");
    console.log(name instanceof String);//returns false
    var color= new String("green");
    console.log(color instanceof String);//returns true

Here the first one is returning false and the second one is returning true,what is the reason if i use variable as name it showing false and are there are any variables like name which throws error as happened with variable name

`window.name` has a special meaning in JavaScript. It shouldn't be used as a global variable. — 4castle, Feb 13 '17 at 05:20
@JLRishe It's listed under *"JavaScript Objects, Properties, and Methods"* — 4castle, Feb 13 '17 at 05:23

score 2 · Answer 1 · answered Feb 13 '17 at 05:20

2

Since name is reserved keywrd in javascript (not really reserved but global object), it directly points to window.name

you can try _name and it will work

var _name = new String("green");
console.log(_name instanceof String);//returns true

answered Feb 13 '17 at 05:20

Dhaval Marthak

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JLRishe · Answer 2 · 2017-02-13T08:02:32.010

This is because you are trying to overwrite the global name variable, which has a setter that automatically converts anything you assign to it to a string (new String creates a String object, which is not the same as a string).

The solution: use a different variable name or properly scope your variables.

console.log(typeof name); // string

var name = new String("green");

console.log(typeof name); // still string

var color = new String("green");

console.log(typeof color); // object

// create new scope
function myFunction() {
  var name = new String("green");

  console.log(typeof name); // object

  var color = new String("green");

  console.log(typeof color); // object
}

myFunction();

brk · Answer 3 · 2017-02-13T05:41:14.133

The instanceof operator tests whether an object has in its prototype chain the prototype property of a constructor.

In the first case it checks the prototype chain find it undefined and return false. This is not only with name but following example will also return false

var simpleStr = 'This is a simple string'; 
console.log(simpleStr instanceof String);

An alternative way is to test it using typeof or constructor The same example will return true for the following case

var simpleStr = 'This is a simple string'; 
simpleStr .constructor == String

For your example also if you do

var name = new String("green");
name .constructor == String

it will return true

This question isn't a problem with OP's understanding of the `instanceof ` operator. OP is asking why it behaves differently on two variables that have both had `new String('...')` assigned to them. — JLRishe, Feb 13 '17 at 05:39

JavaScript instanceof operator returning false

3 Answers3