Cannot read property 'emit' of undefined when trying to emit a document - reactjs

Question

I am trying to create a design for tags of entities in PouchDB with ReactJS. I managed to save my design using the put function, but when I query my design, the response is just an empty array and I am getting following error in console:
TypeError: Cannot read property 'emit' of undefined
I think the problem is in my function that I later use as a map parameter to my design variable:
function emitTagsMap(doc)
{
if (doc !== undefined)
{
if (Array.isArray(doc.tags))
{
doc.tags.forEach(x =>
{
/* Here is probably the problem - this.db is undefined */
this.db.emit(x, null);
});
}
}
};
this.db is declared in constructor:
constructor(service, name)
{
if (!service || !name) throw new Error("PouchDatabase initialized incorrectly");
this.name = name;
this.db = new PouchDB(name);
this.service = service;
this.tagsView();
}
Please bare in mind that I am completely new to PouchDB.
Any ideas how can I initialize the emit function?
Thank you in advance.

Answer 1

I assume, that your function is a part of a JavaScript class (otherwise you have to explain the idea with this). In ES6, you have to bind this to your regular functions. You have two options:
First - bind it via constructor:
constructor() {
this.emitTagsMap = this.emitTagsMap.bind(this);
}
Second - declare the function as an arrow one. This way, react will bind it for you:
emitTagsMap = (doc) =>
{
if (doc !== undefined)
{
if (Array.isArray(doc.tags))
{
doc.tags.forEach(x =>
{
/* Here is probably the problem - this.db is undefined */
this.db.emit(x, null);
});
}
}
};

Answer 2

You don't need to call emit over the database object.
Try this:
function emitTagsMap(doc)
{
if (doc !== undefined)
{
if (Array.isArray(doc.tags))
{
doc.tags.forEach(x =>
{
emit(x, null);
});
}
}
};
According to the PouchDB docs a design document is formed like this:
// first create a new design doc and pass your map function as string into it
var ddoc = {
_id: "_design/my_index",
views: {
by_name: {
map: "function (doc) { if (doc !== undefined) { if (Array.isArray(doc.tags)) { doc.tags.forEach(x => { emit(x, null); }); } } }"
}
}
};
// save it
db.put(ddoc).then(function () {
// success!
}).catch(function (err) {
// some error (maybe a 409, because it already exists?)
});
//Then you actually query it, by using the name you gave the design document when you saved it:
db.query('my_index/by_name').then(function (res) {
// got the query results
}).catch(function (err) {
// some error
});
https://pouchdb.com/guides/queries.html