为什么 array.length + 1 返回 NaN [关闭]

Question

我正在尝试使用下面的 for 循环添加我从 array.length 获得的长度值，但它一直返回 NaN

lent;
 ndat:any;

constructor(private afs: AngularFirestore) {
 this.afs.collection('Courses').doc(`${firebase.auth().currentUser.uid}`).collection('courses').valueChanges().subscribe(data => {

      console.log('my data',data); 
      this.ndat= data;

  this.lent=this.ndat.length;
  console.log('my lenthyyyyy',this.lent);

  });

 for (let i = 0; i < this.dat.length; i++) {
      console.log(i+this.lent);
      this.ref2=this.afs.collection('Courses').doc(`${firebase.auth().currentUser.uid}`).collection('courses').doc(`Course ${this.lent+i}`);
        this.ref2.set({
          name: this.dat[i],
          progress:'In Progress',
          duration:'time'


       });
       }
}

Answer 1

好像 array.length 没有定义。重现：

var array = {}; // no length property
console.log(array.length + 1); // NaN

你在哪里设置'array'和'dat'？