Error trying to diff '[object Object]' in Angular

Question

Looks like something wrong with freight variable in HTML:

Error in app/freightList.component.html:13:8 Error trying to diff '[object Object]'

Below is code

freight.service.ts
=======================

    getFreight (): Promise<Freight[]> {
        return this.http.get(this.freightUrl)
                  .toPromise()
                  .then(this.extractData)
                  .catch(this.handleError);
    }

  private extractData(res: Response) {
      let body = res.json();
      return body || { };
  }

freightList.component.ts
========================
    getFreight() {
        this.freightService
            .getFreight()
            .then(freight => this.freight = freight)
            .catch(error => this.error = error); // TODO: Display error message
    }

freightList.component.html
============================

       <tr *ngFor="let frght of freight">
       <td>{{frght.grp}} - {{frght.grpname}}</td>
       <td>{{frght.subgrp}} - {{frght.subgrpname}}</td>
       <td>{{frght.prodno}} - {{frght.prodname}}</td>
       <td>{{frght.percent}}</td>
       <td>{{frght.effective_date}}</td>
       <td><button (click)="deleteFreight(frght, $event)" class="btn btn-danger btn-sm"><span class="glyphicon glyphicon-remove"></span> Remove</button></td>
       <td><button (click)="editFreight(frght)" class="btn btn-warning btn-sm"><span class="glyphicon glyphicon-edit"></span> Edit</button></td>
       </tr>

Response body
==================

    [{
        "effective_date": "01/01/2016",
        "grp": "01",
        "grpname": "STOPS/FLEX HOSES/COVER PLATES",
        "id": "1",
        "percent": "10",
        "prodname": "DWV PVC PIPE 100MM X 6MTR SN6  SWJ",
        "prodno": "1400200",
        "subgrp": "02",
        "subgrpname": "DWV PIPE - UP TO 150MM"
    }, {
        "effective_date": "01/02/2016",
        "grp": "01",
        "grpname": "STOPS/FLEX HOSES/COVER PLATES",
        "id": "2",
        "percent": "11",
        "prodname": "PVC PIPE    100MM X 6MTR SWJ SN10",
        "prodno": "1400201",
        "subgrp": "03",
        "subgrpname": "DIMPLEX BATHROOM ACCESSORIES"
    }]

Answer 1

The problem (for me) was in my newState definition. Below is the correct definition:

const newState = (state, newData) => {
    return Object.assign([], state, newData);
}

My newState was converting my array to an object because I was using the wrong brackets - the Wrong Way is below.

const newState = (state, newData) => {
    return Object.assign({}, state, newData);
}

Good luck, I hope that helps, Rob

Answer 2

Your extractData (and possibly also your HTTP API) is returning an object {} - ngFor requires an array [] to iterate.

Change your service/API to produce an array, or transform the object in your component.

Answer 3

I ran into this issue when I changed the WebApi I was calling to return a Task<IActionResult> instead of the previous IEnumerable<object>. Check the response isn't wrapped in an object. I had to change my extractData method to:

private extractData(res: Response) {
   let body = res.json();
   return body.result || body || { };
}