Mastering JavaScript Type Detection: typeof, instanceof, constructor & $.type

Introduction

JavaScript defines five primitive data types—Undefined, Null, Boolean, Number, and String—and one complex type, Object. Objects include specific structures such as Array, Function, and Date. The article sets up several test variables representing these common types to demonstrate detection methods.

01. Using typeof to Detect Types

The typeof operator is the most frequently used way to check a variable's type. It correctly identifies Number, String, Boolean, Function, and Undefined, but treats arrays, JSON objects, dates, regular expressions, and errors as object. It also cannot distinguish between null and object.

02. Using instanceof to Detect Types

The instanceof operator, introduced in ECMAScript, determines whether an object is an instance of a specific constructor. Unlike typeof, it can differentiate between built‑in objects such as Array and custom classes, but it still fails to detect primitive types like Number, String, and Boolean.

Additionally, instanceof works only with objects created in the current execution context; objects from different frames or windows have distinct constructor references, leading to false negatives.

03. Using constructor to Detect Types

Every object inherits a constructor property that points to its creating function. By comparing obj.constructor with built‑in constructors (e.g., Number, String), one can identify most types except null and undefined. However, the constructor property can be overwritten, which may produce inaccurate results.

04. Using Object.prototype.toString.call

This method returns a string like [object Array]. The first part always indicates Object, while the second part reveals the actual type. By extracting the second token, developers can reliably detect any JavaScript type, including null and undefined.

05. jQuery’s $.type Implementation

jQuery provides $.type, which internally uses Object.prototype.toString.call to obtain the type string and then maps it to a lowercase type name (e.g., boolean, array). For objects and functions, it falls back to typeof. The article shows the relevant jQuery source code.