Step 3 / Select topic TypeScript Easy

What this bank covers

• Counting assignment type errors
• Mixed array literal inference
• Runtime typeof an array
• Inferred type of a let variable
• as const on an array literal
• Literal widening with let
• Narrowing a union with typeof
• Inference through reduce
• Literal widening in let declarations
• typeof narrowing on string | number
• else branch narrows away the checked member
• Switching on a discriminant property
• The `in` operator as a narrowing check
• Array.isArray as a type guard
• instanceof narrowing for class types
• Truthiness narrowing removes undefined
• Members must exist on every union member
• Equality narrowing on a literal union
• Narrowing an optional parameter with typeof "undefined"
• Interface declaration merging
• Type aliases cannot be redeclared
• Conflicting merged members
• Structural compatibility of interfaces and aliases
• Classes can implement type aliases
• Interfaces describe object shapes only
• What an interface may extend
• Merged interfaces at runtime
• Interfaces do not merge with type aliases
• Interfaces extending type aliases
• Generic identity function
• Explicit type argument must match the argument
• Constraining a type parameter with extends
• Generic classes
• Numeric constraints on type parameters
• keyof constraints reject unknown keys
• Default type parameters
• Generic interfaces with two type parameters
• Multiple type parameters in tuples
• Generic function with a fallback value
• Required strips optional modifiers
• Pick keeps only the named keys
• Omit removes the listed keys
• ReturnType extracts a function's result type
• Parameters yields a tuple of argument types
• Partial makes every property optional
• Readonly blocks property assignment
• Record builds an object type from keys and a value type
• Exclude removes members from a union
• Awaited unwraps nested promises
• Literal inference for const
• Assigning outside a literal union
• Returning literal union members
• Branching on string literal types
• Literal default parameter
• Boolean literal types
• as const on an object
• unknown blocks property access
• never as a return type
• never is assignable to everything
• narrowing unknown with typeof
• never for exhaustiveness checks
• any is not assignable to never
• any disables property checking
• narrowed unknown satisfies the return type
• any, unknown and never assignability
• Parameter properties
• private members
• Inheritance and super()
• Getters
• Static members
• Calling super methods
• Abstract classes
• Class field types
• readonly class fields
• Returning `this` from class methods
• Numeric enum auto-numbering
• Auto-increment after an explicit enum initializer
• Reverse mapping in numeric enums
• String enums have no reverse mapping
• const assertion on an array literal
• String enum member values
• const bindings cannot be reassigned
• Numeric enum members behave as numbers
• Class names are types, instances are plain objects
• Interfaces are erased before runtime
• Numeric enums survive compilation
• Casts are compile-time only
• Generics do not exist at runtime
• Enum values vs erased interfaces
• Optional properties are not created at runtime
• The `!` operator checks nothing at runtime
• `readonly` is enforced only by the compiler
• Type assertions are erased at runtime
• Literal widening in object literals
• Assigning to a literal union
• Literal widening through let
• Indexing an as const object with a literal union
• Narrowing an `unknown` parameter
• Computed numeric enum members
• Computed enum members and reverse mapping