Callbacks

Callback-heavy code is supported where receiver type, overload selection, and delegate shape can be resolved deterministically.

Common supported situations

• JS-surface array callbacks
• Node-style request and event handlers
• CLR delegate calls through generated binding packages
• Express middleware and route handlers

Practical advice

• annotate callback return types when a call surface is sensitive
• prefer explicit numeric/value intent in CLR-heavy callback flows
• treat diagnostics as real surface mismatches, not optional warnings

Example:

const ys = Enumerable.Where(xs, (x: number): boolean => x > 1);

That explicit boolean return often makes the intended overload family obvious to the compiler.

Common sources of trouble

• overloaded CLR methods with weakly inferred callback returns
• callbacks crossing package or generic boundaries without enough type context
• JS-surface callbacks passed into CLR-heavy APIs without explicit intent
• callbacks that rely on dynamic any / unknown escape hatches

Current rule of thumb

• JS-to-JS callback flows are usually straightforward
• CLR-bound callback flows are where explicit annotations matter most
• if the callback crosses package, generic, or overload boundaries, be more explicit than you would in ordinary TypeScript