Function DocumentReference.set() called with invalid data. Unsupported field value: a custom Budget object

You have to transform your array of budgets into an array of pure JavaScript objects.

First step:

const budgets = arrayOfBudget.map((obj)=> {return Object.assign({}, obj)});

Second step:

const proj: Project = {
      id: data.id,
      name: data.name,
      budgetList: budgets
    }

Then you are good to go.

By the way, when developing with a language that compiles to JavaScript you cannot use custom Objects. Instead, you have to use pure JavaScript objects to save in the Firestore Database.

For example, let's say you have this class below:

export class User {
        id: string;
        name: string;
    }

And you try to execute the following code:

 const user = new User();   
 this.db.collection('users').doc().set(user)

You will get an error like:

invalid data. Data must be an object, but it was: a custom User object

Now if you try to execute this other line of code:

 this.db.collection('users').doc().set(Object.assign({}, user))

You will see that your object was saved in the database. Basically Object.assign does the same thing as:

this.db.collection('users').doc().set({id: user.id , name: user.name})

So make use of Object.assign, it will save you a lot of time.

UPDATE

As I have pointed out in a comment below, you can find what documentation says about Custom objects here. As you can see, there is a warning saying:

// Web uses JavaScript objects

Below there is a screenshot of what the documentation says.

You can stringify the object and parse to object again to remove class names of the object.

const budgets = this.budgetProvider.createBudgets(JSON.parse(JSON.stringify(data.budgetList)), projectId);