• FAQ (For Issue Filers)
• Common Feature Requests
• Existing Common Requests
• Behavior That Looks Wrong (And Arguably Is) But Is Currently Working As Intended
• Pre-Declined Feature Requests
• New Utility Types
• Add a Key Constraint to Omit
• Module Specifier Rewriting
• Additional Logic in noUncheckedIndexedAccess
• throws / Checked Exceptions / Typed Exceptions
• Common Misconceptions
• Primitives are { }, and { } Doesn't Mean object
• { } Does Not Refer to Objects With No Properties
• Evolving let and Evolving Arrays Aren't any
• (Indirect) Excess Properties Are OK
• Number.isFinite and Number.isNaN are Typed Correctly
• Parameter Contravariance is Correct
• Parameter Arity Variance is Correct
• void and undefined are Different
• Exclude Isn't Type Negation
• as is the Casting Operator, So it Casts
• The ECMAScript Spec is Descriptive, not Normative
• exclude in tsconfig.json Only Filters include
• The "lib" in skipLibCheck Refers To .d.ts Files
• Non-Invariants
• Circularity Errors May Occur In The Presence of Circularities
• Comment Preservation Not Guaranteed
• Structural vs Instantiation-Based Inference
• Turn On This Flag To Do That
• Assume Array Access Might Be Out of Bounds: noUncheckedIndexedAccess
• Require Properties to Either Be Missing or Not undefined: exactOptionalPropertyTypes
• Enforce readonly in Subtyping / Assignability
• Common Comments
• What Kind of Feedback Are You Looking For?
• Time Marches On
• Can I Work On This?
• Any Updates?
• This Is Closed, But Should Be Open, Or Vice Versa
• Other FAQs and Errors
• Why Method Bivariance?
• The inferred type of "X" cannot be named without a reference to "Y". This is likely not portable. A type annotation is necessary
• FAQ Archive
• Common "Bugs" That Aren't Bugs
• Common Feature Requests
• Type System Behavior
• What is structural typing?
• What is type erasure?
• Why are function parameters bivariant?
• Why are functions with fewer parameters assignable to functions that take more parameters?
• Why are functions returning non-void assignable to function returning void?
• Why are all types assignable to empty interfaces?
• Can I make a type alias nominal?
• How do I prevent two types from being structurally compatible?
• How do I check at run-time if an object implements some interface?
• Why doesn't this incorrect cast throw a runtime error?
• Why don't I get type checking for (number) => string or (T) => T?
• Why am I getting an error about a missing index signature?
• Why am I getting Supplied parameters do not match any signature error?
• Classes
• Why do these empty classes behave strangely?
• When and why are classes nominal?
• Why does this get orphaned in my instance methods?
• What's the difference between Bar and typeof Bar when Bar is a class?
• Why do my derived class property initializers overwrite values set in the base class constructor?
• What's the difference between declare class and interface?
• What does it mean for an interface to extend a class?
• Why am I getting "TypeError: [base class name] is not defined in __extends?
• Why am I getting "TypeError: Cannot read property 'prototype' of undefined" in __extends?
• Why doesn't extending built-ins like Error, Array, and Map work?
• Generics
• Why is A<string> assignable to A<number> for interface A<T> { }?
• Why doesn't type inference work on this interface: interface Foo<T> { }?
• Why can't I write typeof T, new T, or instanceof T in my generic function?
• Commandline Behavior
• Why did adding an import or export modifier break my program?
• What does the error "Exported variable [name] has or is using private name [name]" mean?
• tsconfig.json Behavior
• How can I specify an include?
• Why am I getting the error TS5055: Cannot write file 'xxx.js' because it would overwrite input file. when using JavaScript files?
• Glossary and Terms in this FAQ
• Dogs, Cats, and Animals, Oh My
• "Substitutability"
• Trailing, leading, and detached comments
• Dead Links Parking Lot
• Why is a file in the exclude list still picked up by the compiler?