← Errors
TypeScript reports this error when a type argument does not satisfy the constraints declared for a generic type parameter. Constraints are declared with 'extends' and limit which types can be used as type arguments.
Why it happens
- •Accessing a property on a generic type T without constraining T to have that property.
- •Passing a type argument that doesn't implement the required interface.
- •Using conditional types or mapped types that require specific constraints.
Examples
JavaScript
Code that triggers the error
function getLength<T>(value: T): number {
return value.length; // T has no 'length' constraint
}Error output
error TS2339: Property 'length' does not exist on type 'T'.
How to fix it
Add an extends constraint to the generic parameter: <T extends SomeInterface>. Narrow the constraint to the minimum shape required. If the function should accept any type, avoid accessing specific properties and use type guards instead.
Fixed code
function getLength<T extends { length: number }>(value: T): number {
return value.length; // works for arrays, strings, etc.
}Related errors
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