我有一段代码,其中包含三个功能。一个函数是go(),另一个函数是checkIfUserExists(),另一个函数是userExistsCallback()。 go函数调用checkIfUserExists函数。内部人员checkIfUserExists函数我调用了一个Firebase函数,然后需要调用userExistsCallback()。但是我无法从该Firebase函数内部访问userExistsCallback。
async go() {
var userId = 'ada';
this.checkIfUserExists(userId); // this is working. It perfectly calls the function
console.log('the go function');
}
async userExistsCallback(userId, exists) {
if (exists) {
console.log(' exists!');
} else {
console.log(' does not exist!');
}
console.log('function userExistsCallback ends');
}
async checkIfUserExists(userId) {
var usersRef = firebase.database().ref("news/");
usersRef.child(userId).once('value', function(snapshot) {
var exists = (snapshot.val() !== null);
this.userExistsCallback(userId, exists); // this is not working.
console.log('function checkIfUserExists');
});
}
答案 0 :(得分:1)
this
不起作用,因为它引用了封闭的function
,在本例中为您的once
回调。
将您的once
回调更改为一个arrow function,该绑定不会绑定this
,您可以这样做:
async checkIfUserExists(userId) {
var usersRef = firebase.database().ref("news/");
usersRef.child(userId).once('value', (snapshot) => {
// `this` now refers to `window` where your global functions
// are attached.
var exists = (snapshot.val() !== null);
this.userExistsCallback(userId, exists); // this is not working.
console.log('function checkIfUserExists');
});
}