Ionic asset file location for web and devices with angular http

I’m trying to read file from asset. My code is:

import { HttpClient, HttpParams, HttpHeaders, HttpRequest } from "@angular/common/http";

.
.
  assetURL: string = "../assets/";
  androidAssetURL: string = "'../www/assets/";

.
.

public readAssetFile(fileName: string) {
    var url = this.assetURL + fileName;

    if (this.platform.is("cordova") && this.platform.is("android")) {
      url = this.androidAssetURL + fileName;
    }

    this.log("reading data file location:" + url);
    return this.http
      .get(url, { responseType: "text" })
      .first()
      .toPromise();
  }

the location works for web

reading data file location:…/assets/data/about

but on device I get the location

reading data file location:’…/www/assets/data/about

but the real content isn’t read instead, get the HTML below as content

data file:<!DOCTYPE html>
<html lang="en" dir="ltr">

<head>
  <script data-ionic="inject">
    (function(w){var i=w.Ionic=w.Ionic||{};i.version='3.9.2';i.angular='5.2.11';i.staticDir='build/';})(window);
  </script>
  <meta charset="UTF-8">
  <title>Ionic App</title>
  <meta name="viewport"
    content="viewport-fit=cover, width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
  <met
.
.
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