• FAQs
• Common "Bugs" That Aren't Bugs
• Common Feature Requests
• Type System Behavior
• What is structural typing?
• What is type erasure?
• Why are getters without setters not considered read-only?
• 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?
• Functions
• Why can't I use x in the destructuring function f({ x: number }) { /* ... */ }?
• 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?
• Modules
• Why are imports being elided in my emit?
• Why don't namespaces merge across different module files?
• Enums
• What's the difference between enum and const enums?
• Type Guards
• Why doesn't x instanceof Foo narrow x to Foo?
• Why doesn't isFoo(x) narrow x to Foo when isFoo is a type guard?
• Decorators
• Decorators on function declarations
• What's the difference between @dec and @dec()? Shouldn't they be equivalent?
• JSX and React
• I wrote declare var MyComponent: React.Component;, why can't I write <MyComponent />?
• Things That Don't Work
• You should emit classes like this so they have real private members
• You should emit classes like this so they don't lose this in callbacks
• You should have some class initialization which is impossible to emit code for
• External Tools
• How do I write unit tests with TypeScript?
• Commandline Behavior
• Why did adding an import or export modifier break my program?
• How do I control file ordering in combined output (--out)?
• What does the error "Exported variable [name] has or is using private name [name]" mean?
• Why does --outDir moves output after adding a new file?
• tsconfig.json Behavior
• Why is a file in the exclude list still picked up by the compiler?
• 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?
• Comments
• Why some comments are not preserved in emitted JavaScript even when --removeComments is not specified?
• Why Copyright comments are removed when --removeComments is true?
• Glossary and Terms in this FAQ
• Dogs, Cats, and Animals, Oh My
• "Substitutability"
• Trailing, leading, and detached comments
• GitHub Process Questions
• What do the labels on these issues mean?
• I disagree with the outcome of this suggestion