Use let k: keyof T and a for-in loop to iterate objects when you know exactly what the keys will be.The latter is more generally appropriate, though the key and value types are more difficult to work with. The former is appropriate for constants or other situations where you know that the object won't have additional keys and you want precise types. If you want to iterate over the keys and values in an object, use either a keyof declaration ( let k: keyof T) or Object.entries.
Hopefully this doesn't happen in a nonadversarial environment (you should never add enumerable properties to Object.prototype), but it is another reason that for-in produces string keys even for object literals. This code runs fine, and yet TypeScript flags an error in it. Reading through this item again, I'd add that this is all a good reason to consider using an ES6 Map instead of an object to store key/value pairs!
The difficulty results from a combination of the quirks of JavaScript objects and duck typing. Otherwise, if the type guard returns false, the code attempts to call myMethod() on the instance object again, which will throw a runtime error because myMethod() is not a function on instance when it is not of type MyObject.Iterating over the keys and values in an object is a common operation that's surprisingly hard to write without type assertions in TypeScript. If the type guard returns true, the code calls the myMethod() method on the instance object, which will return 30. Next, the code uses the isMyObject type guard to check if instance is of type MyObject. The variable is then reassigned to a new instance of MyObject with x value of 10 and y value of 20. The code then declares a variable instance with an initial value of 10, which is of type unknown. However, in real-world code, the implementation of the type guard would typically include some actual type checking. In this case, the implementation of the type guard is not really doing any checking and simply returns true. The code above defines a type guard function called isMyObject, which takes an unknown value and returns a boolean indicating whether or not the value is of type MyObject.
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