I am trying to use APP_INITIALIZER TO load data from config file. I&#39;m getting following error:

&gt; &quot;Unhandled Promise rejection: this.appInits[i] is not a function ;
&gt; Zone: &lt;root&gt; ; Task: Promise.then ; Value: TypeError: this.appInits[i]
&gt; is not a function&quot;

Code:

    import { Injectable } from &#39;@angular/core&#39;;
    import { Http } from &#39;@angular/http&#39;;
    import { Observable } from &#39;rxjs/Rx&#39;;
    
    @Injectable()
    export class ApiConfig {
    
      private urlList: any = null;
    
      constructor(private http: Http) {
    
      }
    
      public getUrl(key: any) {
        return this.urlList[key];
      }
    
      public loadConfig(){
    
        let retPromise: Promise&lt;any&gt; =  this.http.get(&#39;./assets/config/api-config.json&#39;)
          .map(res =&gt; res.json()).toPromise();
    
        retPromise.then(resp =&gt; this.urlList=resp.urlList);
        //this.urlList = retPromise.urlList;
    
        return retPromise;
      }
    
    }
    
    
    export  function apiConfigProvider(config: ApiConfig) {
       config.loadConfig();
    }


Any help on what I&#39;m doing wrong?

**Edit:**

Adam&#39;s solution fixed the original error. But now I see a new error:
&gt; TypeError: Cannot set property &#39;urlList&#39; of undefined

As I understand there is no instance of ApiConfig created. But all the examples I see in loading the config file in Angular give same example. I wonder how to force the creation of ApiConfig class. And how I can make it a singleton?
Below is the usage of ApiConfig.

    export BaseService { 
    constructor (private config:ApiConfig){} 
    
    public GetUrl (key: string) { 
       return config.urlList[key]; 
    } 
    
    }

**Answer:**
It looks like there is an issue with the way promise is created in above code. I modified my loadconfig() method as given in the link: https://gist.github.com/fernandohu/122e88c3bcd210bbe41c608c36306db9 this resolved my problem. 

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"this.appInits[i]" is not a function