What does [object Object] mean?

Question

I am trying to alert a returned value from a function and I get this in the alert:

[object Object]  

Here is the JavaScript code:

Answer 1

Basics

You may not know it but, in JavaScript, whenever we interact with string, number or boolean primitives we enter a hidden world of object shadows and coercion.

string, number, boolean, null, undefined, and symbol.

In JavaScript there are 7 primitive types: undefined, null, boolean, string, number, bigint and symbol. Everything else is an object. The primitive types boolean, string and number can be wrapped by their object counterparts. These objects are instances of the Boolean, String and Number constructors respectively.

typeof true; //"boolean"
typeof new Boolean(true); //"object"

typeof "this is a string"; //"string"
typeof new String("this is a string"); //"object"

typeof 123; //"number"
typeof new Number(123); //"object"

If primitives have no properties, why does "this is a string".length return a value?

Because JavaScript will readily coerce between primitives and objects. In this case the string value is coerced to a string object in order to access the property length. The string object is only used for a fraction of second after which it is sacrificed to the Gods of garbage collection – but in the spirit of the TV discovery shows, we will trap the elusive creature and preserve it for further analysis…

To demonstrate this further consider the following example in which we are adding a new property to String constructor prototype.

String.prototype.sampleProperty = 5;
var str = "this is a string";
str.sampleProperty;            // 5

By this means primitives have access to all the properties (including methods) defined by their respective object constructors.

So we saw that primitive types will appropriately coerce to their respective Object counterpart when required.

Analysis of toString() method

Consider the following code

var myObj    = {lhs: 3, rhs: 2};
var myFunc   = function(){}
var myString = "This is a sample String";
var myNumber = 4;
var myArray  = [2, 3, 5];

myObj.toString();     // "[object Object]"
myFunc.toString();    // "function(){}"
myString.toString();  // "This is a sample String"
myNumber.toString();  // "4"
myArray.toString();   // "2,3,5"

As discussed above, what's really happening is when we call toString() method on a primitive type, it has to be coerced into its object counterpart before it can invoke the method.
i.e. myNumber.toString() is equivalent to Number.prototype.toString.call(myNumber) and similarly for other primitive types.

But what if instead of primitive type being passed into toString() method of its corresponding Object constructor function counterpart, we force the primitive type to be passed as parameter onto toString() method of Object function constructor (Object.prototype.toString.call(x))?

Closer look at Object.prototype.toString()

As per the documentation, When the toString method is called, the following steps are taken:

1. If the this value is undefined, return "[object Undefined]".
2. If the this value is null, return "[object Null]".
3. If this value is none of the above, Let O be the result of calling toObject passing the this value as the argument.
4. Let class be the value of the [[Class]] internal property of O.
5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

Understand this from the following example

var myObj       = {lhs: 3, rhs: 2};
var myFunc      = function(){}
var myString    = "This is a sample String";
var myNumber    = 4;
var myArray     = [2, 3, 5];
var myUndefined = undefined;
var myNull      = null;

Object.prototype.toString.call(myObj);        // "[object Object]"
Object.prototype.toString.call(myFunc);       // "[object Function]"
Object.prototype.toString.call(myString);     // "[object String]"
Object.prototype.toString.call(myNumber);     // "[object Number]"
Object.prototype.toString.call(myArray);      // "[object Array]"
Object.prototype.toString.call(myUndefined);  // "[object Undefined]"
Object.prototype.toString.call(myNull);       // "[object Null]"

References: https://es5.github.io/x15.2.html#x15.2.4.2 https://es5.github.io/x9.html#x9.9 https://javascriptweblog.wordpress.com/2010/09/27/the-secret-life-of-javascript-primitives/