TypeScript will perform access property checks on any object that has signed an interface contract. Using the interface means that the structure of the object has to be adhere to. It can lead to silent errors. TypeScript treats object literals with caution, and does access property checks. You can see the access property check is done on car three on line 32 at the bottom left hand of the screen will hover over to show you the error. Window property is not known in vehicle, at the top right on line 35. We define another car object without signing the vehicle contract. On line 42 we have a function called Park vehicle that expects to receive an object sign the vehicle contract. Car three could be parked here with no problems, duck typing in action. The same goes for car for more duck typing Passing in an object literal notation produces an error if the object doesn't meet the vehicle interface requirements. This is shown on line 55. Accesses property checking in action again. I submit an object lecture is seen as dangerous and error prone, and iterators could have problems here, with the being too many fields. Also passing in an object literal, it's already been signed is okay and seen as less dangerous, especially if the object passed in meets the contract. All this is not really recommended. We could use type assertion at a type index signature, or assigned to a variable first to get around all of this. But should we be trying to? The answer is probably not as the majority of access property check errors usually end up in being an indication of a book.

Lectures

• An Introduction to Interfaces
• Function Types
• Optionals
• Indexable Types
• Interfaces and Classes
• Extendable Interfaces