Array.length等於0總是

Question

我試圖讓我的數組的長度，但我總是得到0，即使控制檯顯示，我把5個OBJETS

async function getDiaries() { 
return new Promise((resolve, reject) => { 
    var url = firebase.database().ref('/diary'); 
    var diaries = []; 
    url.on('value', (snap) => { 
    snap.forEach((child) => { 
     diaries.push({ 
      key: child.key, 
      name: child.val().name, 
      date: child.val().description, 
      url: child.val().url, 
      idOwner: child.val().idOwner 
     }); 
    }); 
}); 
console.log(diaries.length);// get 0 
resolve(diaries) 
})} 

這是控制檯顯示我

Answer 1

url.on可能是異步的，這意味着你需要做的回調裏面console.log：

async function getDiaries() { 
    return new Promise((resolve, reject) => { 
    var url = firebase.database().ref('/diary'); 
    var diaries = []; 
    url.on('value', (snap) => { 
     snap.forEach((child) => { 
     diaries.push({ 
      key: child.key, 
      name: child.val().name, 
      date: child.val().description, 
      url: child.val().url, 
      idOwner: child.val().idOwner 
     }); 
     }); 
     console.log(diaries.length); // get 0 
     resolve(diaries); 
    }); 
    }); 
} 

Answer 2

將console.log移動到您的回調函數中，否則在執行回調之前，它通常會運行。換句話說：你得到一個0，因爲此時的長度是 0。

Answer 3

Ofcourse，你會得到這個，因爲這是異步功能，你可以嘗試這樣的

async function getDiaries() { 
return new Promise((resolve, reject) => { 
    var url = firebase.database().ref('/diary'); 
    var diaries = []; 
    url.on('value', (snap) => { 
    snap.forEach((child) => { 
     diaries.push({ 
      key: child.key, 
      name: child.val().name, 
      date: child.val().description, 
      url: child.val().url, 
      idOwner: child.val().idOwner 
     }); 
    }); 
    console.log(diaries.length); 
    resolve(diaries); 
}); 
})}