Mastering TypeScript 3 - Third Edition

Union types

We are not quite finished with the declaration file for the ErrorHelper class just yet. If we take a look at the original JavaScript usage of the ErrorHelper class, we will notice that the containsErrors function also allows for the failure property of responseText to be a string:

    var failureMessage = { 
        responseText : { 
            "failure" : "true", 
            "errorMessage" : "Error Message from Typescript" 
        } 
    }  

If we compile this code now, we will get the following compile error:

In the preceding definition of the failureMessageString variable, the type of the "failure" property is "true", which is of type string , and not true, which is of type ...