Cannot use spyon on a primitive value; undefined given

by

When using the “spyOn” function in JavaScript, you cannot use it directly on a primitive value like string, number, boolean, etc. The reason behind this restriction is that spying is used for tracking and modifying behavior of object’s methods, not for primitive values, which do not have methods.

For example, you cannot spy on a string value like “Hello World”:


      const str = "Hello World";
      spyOn(str, 'toUpperCase'); // This will throw an error

In the above example, an error will be thrown because “toUpperCase” is a method of the String object, not a method of the primitive string value.

To overcome this limitation, you can wrap the primitive value in an object and then spy on its methods. Here’s an example:


      const obj = { value: "Hello World" };
      spyOn(obj, 'toString');

In this example, we create an object “obj” with a property “value” that holds the primitive string value. Now, we can spy on its “toString” method successfully.

Alternatively, if you want to spy on a specific function that receives a primitive value as an argument, you can create a separate function to wrap the primitive value and then spy on that wrapper function. Here’s an example:


      function wrapper(value) {
        // Do something with the value
        return value;
      }

      spyOn(wrapper, 'toString');

In this example, we create a function “wrapper” that receives a primitive value as an argument and performs some operations on it. We can then spy on the “toString” method of this wrapper function.

In summary, the “spyOn” function cannot be directly used on primitive values in JavaScript. You need to wrap them in objects or create wrapper functions to spy on their methods or behavior.

Same cateogry post

Leave a comment