What I can suggest is creating a file called API.ts. Inside, you should create an API class with properties and functions similar to API.js, but taking into account the syntax of Typescript.

It seems that the original author intended to use jquery in the file, but it was not imported. So, I would recommend importing it as well. Assuming you have been provided with this file, make sure you place it inside src/assets and modify the import path in my code to match the filename accordingly.

This is what I have for you:

import * as $ from '.assets/jquery.js';

export class API {
  ServerUrl = '';

  Type = {
    Day: 'day',
    Night: 'night'
  };

  Days = {
    Monday: 'Monday',
    Tuesday: 'Tuesday',
    Wednesday: 'Wednesday',
    Thursday: 'Thursday',
    Friday: 'Friday',
    Saturday: 'Saturday',
    Sunday: 'Sunday'
  };

  MinTemperature = parseFloat('5.0');
  MaxTemperature = parseFloat('30.0');
  MaxSwitches = 5;

  Time;
  CurrentDay;
  DayTemperature;
  NightTemperature;
  CurrentTemperature;
  TargetTemperature;
  ProgramState;

  Program:any = {};

  constructor() {
    this.Program[this.Days.Monday]    = [];
    this.Program[this.Days.Tuesday]   = [];
    this.Program[this.Days.Wednesday] = [];
    this.Program[this.Days.Thursday]  = [];
    this.Program[this.Days.Friday]    = [];
    this.Program[this.Days.Saturday]  = [];
    this.Program[this.Days.Sunday]    = [];
  }

  /* Retrieve day program */
  getProgram(day) {
    return this.Program[day];
  }

  /* Sort the heating periods (the periods when the heating is on) and merge overlapping ones */
  sortMergeProgram(day) {
    let program = this.getProgram(day);
    program.sort(function(a, b) {
      return this.parseTime(a[0]) - this.parseTime(b[0])
    });
    for (let i = 0; i < program.length - 1; i++) {
      if (this.parseTime(program[i][1]) >= this.parseTime(program[i + 1][0])) {
        let start = (program[i][0]);
        let end = (this.parseTime(program[i][1]) > this.parseTime(program[i + 1][1])) ? program[i][1] : program[i + 1][1];
        program.splice(i, 2);
        program.push([start, end]);
        this.sortMergeProgram(day);
        break;
      }
    }
  }

  /* Retrieve all data from the server except for weekProgram */
  get(attribute_name, xml_tag) {
    return this.requestData(
      "/" + attribute_name,
      function(data) {
        return $(data).find(xml_tag).text();
      }
    );
  }

  /* Retrieve the week program */
  getWeekProgram() {
    return this.requestData(
    '/weekProgram',
    function(data) {
      $(data).find('day').each(function() {
        let day = (<any>$(this)).attr('name');
        this.Program[day] = [];
        $(this).find('switch').each(function() {
          if ((<any>$(this)).attr('state') == 'on') {
            if ((<any>$(this)).attr('type') == this.Type.Day) {
              this.getProgram(day).push([$(this).text(), '00:00']);
            } else {
              this.getProgram(day)[this.getProgram(day).length - 1][1] = $(this).text();
            }
          }
        })
      });
      return this.Program;
    }
  );
}

  /* Upload all data to the server except for currentTemperature and weekProgram */
  put(attribute_name, xml_tag, value) {
    this.uploadData("/" + attribute_name, "<" + xml_tag + ">" + value + "</" + xml_tag + ">");
  }

  requestData(address, func) {
    let result;
    (<any>$).ajax({
      type: "get",
      url: this.ServerUrl + address,
      dataType: "xml",
      async: false,
      success: function(data) {
        result = func(data);
      }
    });
    return result;
  }

  setWeekProgram() {
    let doc = document.implementation.createDocument(null, null, null);
    let program = doc.createElement('week_program');
    program.setAttribute('state', this.ProgramState ? 'on' : 'off');
    for (let key in this.Program) {
      let day = doc.createElement('day');
      day.setAttribute('name', key);

      let daySwitches = [];
      let nightSwitches = [];

      let i, text, sw;
      let periods = this.getProgram(key);
      for (i = 0; i < periods.length; i++) {
        daySwitches.push(periods[i][0]);
        nightSwitches.push(periods[i][1]);
      }

      for (i = 0; i < this.MaxSwitches; i++) {
        sw = doc.createElement('switch');
        sw.setAttribute('type', this.Type.Day);

        if (i < daySwitches.length) {
          sw.setAttribute('state', 'on');
          text = doc.createTextNode(daySwitches[i]);
        } else {
          sw.setAttribute('state', 'off');
          text = doc.createTextNode('00:00');
        }
        sw.appendChild(text);
        day.appendChild(sw);
      }

      for (i = 0; i < this.MaxSwitches; i++) {
        sw = doc.createElement('switch');
        sw.setAttribute('type', this.Type.Night);

        if (i < nightSwitches.length) {
          sw.setAttribute('state', 'on');
          text = doc.createTextNode(nightSwitches[i]);
        } else {
          sw.setAttribute('state', 'off');
          text = doc.createTextNode('00:00');
        }
        sw.appendChild(text);
        day.appendChild(sw);
      }
    program.appendChild(day);
  }
doc.appendChild(program);
this.uploadData('/weekProgram', (new XMLSerializer()).serializeToString(doc));
}

setDefault() {
let doc = document.implementation.createDocument(null, null, null);
let program = doc.createElement('week_program');
program.setAttribute('state', this.ProgramState ? 'on' : 'off');
for (let key in this.Program) {
  let day = doc.createElement('day');
  day.setAttribute('name', key);

  let daySwitches = [];
  let nightSwitches = [];

  let i, text, sw;

  for (i = 0; i < this.MaxSwitches; i++) {
    sw = doc.createElement('switch');
    sw.setAttribute('type', this.Type.Night);
    sw.setAttribute('state', 'off');
    text = doc.createTextNode('00:00');
    sw.appendChild(text);
    day.appendChild(sw);
  }

  for (i = 0; i < this.MaxSwitches; i++) {
    sw = doc.createElement('switch');
    sw.setAttribute('type', this.Type.Day);
    sw.setAttribute('state', 'off');
    text = doc.createTextNode('00:00');
    sw.appendChild(text);
    day.appendChild(sw);
  }

  program.appendChild(day);
}
doc.appendChild(program);
this.uploadData('/weekProgram', (new XMLSerializer()).serializeToString(doc));
}

uploadData(address, xml) {
(<any>$).ajax({
  type: "put",
  url: this.ServerUrl + address,
  contentType: 'application/xml',
  data: xml,
  async: false
});
}

parseTime(t) {
return parseFloat(t.substr(0, 2)) + parseFloat(t.substr(3, 2)) / 60;
}

/* Add a heating period for a specific day */
addPeriod(day, start, end) {
let program = this.getWeekProgram()[day];
program.push([start, end]);
this.sortMergeProgram(day);
this.setWeekProgram();
}

/* Remove a heating period from a specific day.
 idx is the idex of the period with values from 0 to 4
*/
removePeriod(day, idx) {
let program = this.getWeekProgram()[day];
let start = program[idx][0];
let end = program[idx][1];
program.splice(idx, 1);
this.setWeekProgram();
}

/* Check whether the temperature is within the range [5.0,30.0] */
inTemperatureBoundaries(temp) {
temp = parseFloat(temp);
return (temp >= this.MinTemperature && temp <= this.MaxTemperature);
}
}

Now, you can import the API in any Ionic component:

import { API } from './API'

And then use it in your code as follows:

export class yourPage {
constructor(public api: API){}

yourFunction() {
this.api.setDefault();
}
}