Why Does JavaScript Display '[object Object]' When Logging an Object?

Understanding "[object Object]" in JavaScript

In JavaScript, you may encounter the string "[object Object]" when attempting to alert or log the value of an object. This string represents the default serialization of an object, providing insights into the object's type.

Why "[object Object]" instead of "[object]"?

Unlike some programming languages, JavaScript has various object types. Each type is identified by its constructor function. The following examples illustrate the serialization of different object types:

function stringify(x) {
    console.log(Object.prototype.toString.call(x));
}

stringify(function () {}); // [object Function]
stringify([]); // [object Array]
stringify(/x/); // [object RegExp]
stringify(new Date); // [object Date]
stringify({}); // [object Object]

In JavaScript, the term "object" (lowercase) generally refers to the structural nature of a thing. However, when specifically discussing "objects," developers typically mean "Object objects" (capitalized). This is because the constructor function for these objects is called Object.

Returning an Object from a Function

In your provided code, the function whichIsVisible() returns an object that represents the visible element. When you attempt to alert the return value, JavaScript serializes the object to "[object Object]" because it's an Object object.

To display the actual element rather than the serialized string, you could modify the function to return the element's ID instead:

function whichIsVisible() {
    if (!.is(':hidden')) return .attr('id');
    if (!.is(':hidden')) return .attr('id');
}

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