如何使用angular和firebase从json值获取平均值

Question

first of all, I'm French so all my apologies for my english...

I would like to calculate an average from values in json using firebase and angular. My goal is to filter with a specific id and get the rate value.

"avis" : {
"-LTMQXqMDrRPgJmtYteU" : {
  "date" : "11/11/2018",
  "description" : "jkldjflsdjfklsdjklfjkdkl",
  "id" : "-LPzmMTwf8muvAl8xhPM",
  "name" : "daniel",
  "rate" : "4"
},
"-LTM_fI6KREHqKkdcjc1" : {
  "date" : "12/12/2018",
  "description" : "jlkj",
  "id" : "-LPzmr9M6DrSCViNphj-",
  "name" : "sfgf",
  "rate" : 2
},
"-LWQJuqDJoogUcZt5KPL" : {
  "date" : "17/01/2019",
  "description" : "FFFF",
  "id" : "-LPzmr9M6DrSCViNphj-",
  "name" : "FFF",
  "rate" : 1
},

... For example, just below, I would like to calculate the rate average for id -LPzmr9M6DrSCViNphj-. So I get rate 2 and 1, the average is 1.5.

I create a function :

    this.afDb.list('/avis', ref => ref.orderByChild('id').equalTo(this.item.key)).valueChanges().
subscribe(
  result => {
    console.log('result: ', result);
  });

But when I do that, I have all values and I would like only rate values...

I don't know if it is clear...

Thank you very much for your support !

Answer 1

const total = avis.reduce((sum, item) => sum + item.rate, 0);

console.log(total);

OR

this.http.get('/avis')
.map(response.avis)
.map(res => res.rate)
.reduce((a, b) => a + b)
.subscribe(res => console.log(res))

Answer 2

You can use .map() & reduce() to achieve your avg value.

let listData = this.afDb.list('/avis', {
            query: {
                orderByChild: 'id',
                equalTo: this.item.key,
            }
        }).valueChanges();

let avg = listData.map(res => res["rate"]).reduce((a, b) => a + b)/listData.length;

Or with the same code you can use like below code to get average.

this.afDb.list('/avis', ref => ref.orderByChild('id').equalTo(this.item.key)).valueChanges().
subscribe(
  result => {

    console.log('result: ', result);
    let avg = result.map(res => res. rate).reduce((a, b) => a + b)/result.length;

  });

By .map() & .reduce() you will get your total rating and then divide it with array length .

Ref Link : https://www.jstips.co/en/javascript/array-average-and-median/

Hope this will helps!

Answer 3

@CodeChanger :

this.listData = this.afDb.list('avis', ref => ref.orderByChild('id').equalTo(this.item.key)).valueChanges();
this.avg = this.listData.map(res => res.rate).reduce((a, b) => a + b) / this.listData.length;

There is no error after the compilation but when I launch my app, I have this error:

ERROR Error: Uncaught (in promise): TypeError: this.listData.map is not a function