Host Objects

The well-covered rules for how variables behave in JS have exceptions to them when it comes to variables that are auto-defined, or otherwise created and provided to JS by the environment that hosts your code (browser, etc.) — so called, “host objects” (which include both built-in objects and functions).

For example:

var a = document.createElement( "div" );
typeof a;                                // "object" -- as expected
Object.prototype.toString.call( a );    // "[object HTMLDivElement]"
a.tagName;                                // "DIV"

a is not just an object, but a special host object because it’s a DOM element. It has a different internal [[Class]] value ("HTMLDivElement") and comes with predefined (and often unchangeable) properties.

Another such quirk has already been covered, in the “Falsy Objects” section in Chapter 4: some objects can exist but when coerced to boolean they (confoundingly) will coerce to false instead of the expected true.

Other behavior variations with host objects to be aware of can include:

• not having access to normal object built-ins like toString()
• not being overwritable
• having certain predefined read-only properties
• having methods that cannot be this-overriden to other objects
• and more…

Host objects are critical to making our JS code work with its surrounding environment. But it’s important to note when you’re interacting with a host object and be careful assuming its behaviors, as they will quite often not conform to regular JS objects.

One notable example of a host object that you probably interact with regularly is the console object and its various functions (log(..), error(..), etc.). The console object is provided by the hosting environment specifically so your code can interact with it for various development-related output tasks.

In browsers, console hooks up to the developer tools’ console display, whereas in node.js and other server-side JS environments, console is generally connected to the standard-output (stdout) and standard-error (stderr) streams of the JavaScript environment system process.