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Vue.jsのdataオブジェクトを参照する際に Uncaught (in promise) TypeError: Cannot set property 'foo' of undefined

Posted at

Promiseパターンで返ってくる値を、dataオブジェクトのfooにセットします。

export default {
  name: 'foo',
  data() {
    return {
      foo: ''
    }
  }
...
Promise-pattern(Before)
getData: function() {
  this.$firestore.collection('posts').where('categories.cat', '==', true)
    .get()
    .then(function(querySnapshot) {
      let data = []
      querySnapshot.forEach(function(doc) {
        data.push(doc.data())
      })
      this.foo = data
    })

Uncaught (in promise) TypeError: Cannot set property 'foo' of undefined

undefinedfooには何もセットできんぞ!!」と怒られてしまいます。

解決策: thisへのreference用変数(self)をつくる

Promise-pattern(After)
getData: function() {
  let self = this
  this.$firestore.collection('posts').where('categories.cat', '==', true)
    .get()
    .then(function(querySnapshot) {
      let data = []
      querySnapshot.forEach(function(doc) {
        data.push(doc.data())
      })
      self.foo = data
    })

無事、dataオブジェクトのfooに対して返り値をセットすることができました。:tada:
thisや変数のscope範囲を理解していないとハマります...。

参考リンク

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