javascript - ForEach is faster than my Observable returns values -

i have following code:

evaluatelocations() {   const locs = this.currenttour.locids;   let lastloc = null;   locs.foreach(loc => {     console.log(lastloc, loc);     if (lastloc !== null) {       console.log('if durch', lastloc, loc);       this.afdb.list('routes', {         query: {           orderbychild: 'idstart',           equalto: loc         }       }).switchmap(items => {         console.log('switchmap starts', lastloc, loc); //here output lastloc = loc while shouldn't         const filtered = items.filter(item => item.idend === lastloc);         const route = filtered[0];         return observable.of(route);       }).subscribe();     }     lastloc = loc;   }); } 

now, @ point switchmap comes play, foreach loop seems have run through filtering done wrong.

my logic is:
want loop through array of ids. call database gets entries starting point locs[n]. want filter locs[n+1] , return 1 result.

in theory should work, given query firebase database, observable has time lag in there , cannot values enough. thing suspect , read fact observables might cancelled if 1 started before current 1 finished. when run code not seem problem.

if want work list sequentially, need wait operation of observable finish. idiomatic way seems be converting observable promise , await that:

applying code:

async evaluatelocations() { // <-- make method async, can use await   const locs = this.currenttour.locids;   let lastloc = null;   (let lockey in locs) { // <-- use regular for-in loop     let loc = locs[lockey];     console.log(lastloc, loc);     if (lastloc !== null) {       console.log('if durch', lastloc, loc);       await this.afdb.list('routes', { // <-- use await here         query: {           orderbychild: 'idstart',           equalto: loc         }       }).switchmap(items => {         console.log('switchmap starts', lastloc, loc); //here output lastloc = loc while shouldn't         const filtered = items.filter(item => item.idend === lastloc);         const route = filtered[0];         return observable.of(route);       }).topromise(); // <-- convert promise     }     lastloc = loc;   } } 

don't forget need await evaluatelocations() in calling function, if need afterwards relies on outcome of evaluatelocations().