我的 Javascript 函数不断返回未定义

Question

I have a function,called getIt() , which finds the top 5 venues through the Foursquare API. Inside the getVenues() , i push the name of the first item of the venues in places array, and as it seems it exists normally inside of it. However when I call the function and set it to the variable place, I get undefined . What's wrong?

 var place=getIt(); console.log(place[0]); function getIt(){ var params={ "near":"Athens,Gr", "query":"sushi", "limit":5 } var places=[]; foursquare.getVenues(params, function(error, venues) { console.log(venues.response.venues[0].name); places.push(venues.response.venues[0]) console.log(places[0].name); }); //console.log(venues.response.venues[0]); return places; }

Answer 1

Your code is asynchronous. It means that the function you use does not block the execution of the code following after it. You essentially send a request, but do not wait for it, you just say "when response arrives, populate this array with the results, if error did not occur, otherwise handle the error (which you should)". In your getIt function you create an array and actually return it rightaway. It will be populated later. But immediately after the execution of the getIt function the places array is empty, thus its first element is undefined . You have two options:

Do your manipulations inside the callback, or provide callback to the getIt function:

function getIt(params, callback) {
   foursquare.getVenues(params, function(error, venues) {
   if (!error) callback(venues)
});
}
 getIt(someParams, function(venues) {// handle venues here})

The other option is the async keyword. If you use a version of Javascript that supports it (ES6 or higher, if I remember correctly), and the foursquare API has methods returning a Promise you can give it a try