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4.9.4 The instanceof Operator

4.9.4 The instanceof Operator


The instanceof operator expects a left-side operand that is an object and a right-side operand that identifies a class of objects. The operator evaluates to true if the left-side object is an instance of the right-side class and evaluates to false otherwise. Chap ter 9 explains that, in JavaScript, classes of objects are defined by the constructor function that initializes them. Thus, the right-side operand of instanceof should be a function. Here are examples:

var d = new Date(); // Create a new object with the Date() constructor d instanceof Date; // Evaluates to true; d was created with Date() d instanceof Object; // Evaluates to true; all objects are instances of Object d instanceof Number; // Evaluates to false; d is not a Number object var a = [1, 2, 3]; // Create an array with array literal syntax a instanceof Array; // Evaluates to true; a is an array a instanceof Object; // Evaluates to true; all arrays are objects a instanceof RegExp; // Evaluates to false; arrays are not regular expressions

Note that all objects are instances of Object. instanceof considers the “superclasses” when deciding whether an object is an instance of a class. If the left-side operand of instanceof is not an object, instanceof returns false. If the right-hand side is not a function, it throws a TypeError.

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