Service code combines types with policies: how many times a job may run, which fields a public record exposes, and whether a message is delivered after a subscriber leaves. A useful type narrows the possible mistakes, but the implementation still has to enforce those policies.
This set revisits the emitter, queue, validator, declaration adapters, unknown errors, public records, result unions, and exhaustive status handling. Each exercise is independent, so reset your assumptions when you move to another scenario. All input stays local. The hidden checks focus on boundaries that tend to escape a happy-path demonstration: duplicate identifiers, missing fields, empty text, exhausted budgets, and values that are structurally plausible but have the wrong runtime type. No new service capability is required.
type Listener<T>={id:string;fn:(value:T)=>void;remaining:number};
class Emitter<E extends Record<string,unknown>> {
private entries=new Map<keyof E,Listener<unknown>[]>();
on<K extends keyof E>(event:K,id:string,fn:(value:E[K])=>void,remaining=Infinity):void { const rows=this.entries.get(event) ?? []; rows.push({id,fn:fn as (value:unknown)=>void,remaining}); this.entries.set(event,rows); }
off<K extends keyof E>(event:K,id:string):void { this.entries.set(event,(this.entries.get(event) ?? []).filter(row=>row.id!==id)); }
count<K extends keyof E>(event:K):number { return (this.entries.get(event) ?? []).length; }
clear<K extends keyof E>(event:K):void { this.entries.delete(event); }
emit<K extends keyof E>(event:K,value:E[K]):void { for(const row of [...(this.entries.get(event) ?? [])]) { row.remaining-=1; if(row.remaining===0) this.off(event,row.id); row.fn(value); } }
}
function solve(commands:string[]):string[] { const out:string[]=[]; const bus=new Emitter<Record<string,string>>(); for(const command of commands){ const [op,topic,id,extra]=command.split("|"); if(op==="on") bus.on(topic,id,v=>out.push(id+":"+v));
else if(op==="emit") bus.emit(topic,id);
else if(op==="off") bus.off(topic,id);
else if(op==="once") bus.on(topic,id,v=>out.push(id+":"+v),1);
else if(op==="count") out.push(String(bus.count(topic)));
else if(op==="clear") bus.clear(topic);
else if(op==="prefix") bus.on(topic,id,v=>out.push(id+":"+extra+v));
else if(op==="many") for(const item of id.split(",")) bus.emit(topic,item);
else if(op==="limit") bus.on(topic,id,v=>out.push(id+":"+v),Number(extra)); } return out; }
A complete implementation for the first scenario; trace its input before editing.