I'm trying to get all the posts from Firebase and add them to an array, but I'm getting the following error:
Uncaught (in promise) TypeError: Cannot read property 'blogItems' of undefined
The following is the script:
export default {
data(){
return{
blogItems: []
}
},
mounted(){
this.getPosts();
},
methods:{
getPosts(){
database.collection('blog').doc('yP6aYXvisFbTsqtQ3MEfuyz6xYE3').collection('posts').get().then(snapshot =>{
const posts = snapshot.docs.map(doc => doc.data())
posts.forEach(function(post){
this.blogItems.push(post.content)
})
})
},
}
} 
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In your case, change
thisinthis.blogItemstoposts.You have two solutions
Store
thisinto a variable and use itgetPosts(){ let tis = this; database.collection('blog').doc('yP6aYXvisFbTsqtQ3MEfuyz6xYE3').collection('posts').get().then(snapshot =>{ const posts = snapshot.docs.map(doc => doc.data()) posts.forEach(function(post){ tis.blogItems.push(post.content) }) }) },Use arrow functions
getPosts(){ database.collection('blog').doc('yP6aYXvisFbTsqtQ3MEfuyz6xYE3').collection('posts').get().then(snapshot =>{ const posts = snapshot.docs.map(doc => doc.data()) posts.forEach((post) => { this.blogItems.push(post.content) }) }) },