Why Do String, Boolean, and Number Literals Fail the `instanceof` Test?

Instanceof Conundrum: Why Some Literals Fail to Match

Despite being fundamental to JavaScript, the instanceof operator can exhibit puzzling behavior when used with certain literals. Notably, string, Boolean, and Number literals all evaluate to false when tested against their respective constructors.

<code class="js">"foo" instanceof String //=> false
"foo" instanceof Object //=> false

true instanceof Boolean //=> false
true instanceof Object //=> false
false instanceof Boolean //=> false
false instanceof Object //=> false

12.21 instanceof Number //=> false
/foo/ instanceof RegExp //=> true</code>

This behavior is surprising, as one would expect literals to be instances of their corresponding types. However, the reason for their inconsistency lies in the distinction between primitives and objects.

Primitivism at Play

JavaScript primitives, such as strings, numbers, and booleans, are built-in types that are not created through object constructors. Instead, they are fundamentally different from objects constructed within JavaScript.

<code class="js">var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral";
color2 instanceof String; // returns false (color2 is not a String object)</code>

As demonstrated above, using the String constructor creates a String object, which will evaluate to true when tested with instanceof. However, the primitive string literal "coral" does not inherit this behavior.

Alternative Type-Checking Methods

The puzzling behavior of instanceof with primitives has led to the adoption of alternative methods for type-checking:

• typeof Operator: The typeof operator returns the primitive type of a variable, e.g., typeof "foo" === "string".
• Object.prototype.toString: This method can also be employed for type-checking, e.g., Object.prototype.toString.call("foo") === "[object String]".

In conclusion, the behavior of instanceof with primitives is a result of JavaScript's unique type system. While this can lead to confusion, understanding the distinction between primitives and objects can help developers navigate these nuances effectively.

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