Declaring what things are.
WHAT CAN BE ANNOTATED
Variables Function parameters Return values Object shapes
WHAT THE BASIC TYPES ARE
Text, numbers and boolean values Arrays of a given type Objects with defined properties The absence of a value, in its two forms A type meaning anything
WHAT TO AVOID
That last one.
WHY
It disables checking entirely, which defeats the purpose.
WHAT TO USE INSTEAD WHEN THE TYPE IS GENUINELY UNKNOWN
The type requiring you to narrow it before use.
WHY
It forces a check, rather than allowing assumptions.
WHAT INFERENCE DOES
Determines types automatically from assignment.
WHAT THAT MEANS
Annotating everything is unnecessary.
WHAT TO ANNOTATE EXPLICITLY
Function parameters and return types Anything where inference produces something too broad
WHY FUNCTION SIGNATURES
They are the contract, and they document intent.
WHAT UNION TYPES PROVIDE
A value that may be one of several types.
WHAT THAT REQUIRES
Narrowing before use, which the compiler enforces.
