Cannot spyon on a primitive value; undefined given

The error message “cannot spy on a primitive value; undefined given” typically occurs when attempting to spy on a property or method of a primitive value in JavaScript.

In JavaScript, primitive values include strings, numbers, booleans, null, and undefined. These values are immutable, which means their properties or methods cannot be spied on since they do not have any. The error message specifically mentions “undefined,” indicating that an attempt was made to spy on an undefined value.

Here’s an example to illustrate this:

    const name = "John";
    spyOn(name, 'toLowerCase'); // Throws the "cannot spy on a primitive value; undefined given" error
  

In the above example, we have a string primitive value “name” assigned to “John.” Then, we try to spy on the “toLowerCase” method of “name”. However, since “name” is a primitive string value, it does not have any methods, leading to the error.

To resolve this issue, you need to ensure that the value you are spying on is not a primitive value. Instead, you should spy on properties or methods of objects or functions. Here’s an example:

    const person = {
      name: "John",
      greet: function() {
        console.log("Hello, " + this.name + "!");
      }
    };

    spyOn(person, 'greet'); // Successful spy

    person.greet();
  

In this example, we have an object “person” with a “name” property and a “greet” method. We successfully spy on the “greet” method and can later invoke it using “person.greet()”.