What's the Best Way to Determine if a JavaScript Variable is an Array?

Determining Array Type in JavaScript

In JavaScript, classifying variables as arrays can be a crucial task for data manipulation and processing. This can be accomplished through various methods, each with its advantages and drawbacks.

Constructor Method

The recommended approach is using the constructor property, which provides the most efficient and reliable way to identify arrays:

variable.constructor === Array

This checks if the variable's constructor (i.e., the object that created it) is indeed the Array object. This method is highly optimized by JavaScript engines.

Array.isArray() Method

Another built-in method specifically designed for this purpose is Array.isArray():

Array.isArray(variable)

This method explicitly determines whether a variable is an array type, making it a reliable option. However, it may not be as efficient as the constructor method.

instanceof Operator

The instanceof operator can be used to check if a variable is an instance of an Array object:

variable instanceof Array

However, this method may not be as fast as the constructor method, as it involves additional object-oriented concepts.

Object.prototype.toString() Method

A more comprehensive approach that can detect various types involves using the toString() method on the Object prototype:

Object.prototype.toString.call(variable) === '[object Array]'

While this method can be applied to any type, it's not as efficient as the constructor method when specifically checking for arrays.

Other Considerations

When dealing with object properties that may be arrays, it's essential to first check if the property exists before attempting to verify its type:

variable.prop && variable.prop.constructor === Array

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