B4X: ABMaterials B4JS – 05 JSON & Ajax Calls


I’ve put JSON and Ajax calls in one tutorial, as in many cases they go together anyway.

The JSONParser and JSONGenerator in B4JS are almost identical as their B4X versions. The only method not supported (yet) is NextValue. To be honest, I haven’t needed it in any of my projects yet so if someone knows a good example of its usage, please let me know.

This example is based on Erels example, without the reading from file part as this is not supported in B4JS.

public Sub TestJson()
   ' String -> JSON
   Dim JSON As JSONParser
   JSON.Initialize($"{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}

   ' exploring the JSON tree
   Dim Map1 As Map
   Map1 = JSON.NextObject
   Dim m As Map 'helper map for navigating
   Dim MenuItems As List
   m = Map1.Get("menu")
   m = m.Get("popup")
   MenuItems = m.Get("menuitem")
   For i = 0 To MenuItems.Size - 1
       m = MenuItems.Get(i)

   ' manipulating and generating a new JSON file
   Dim Data As List
   Data.Add(Map1) 'add the previous map loaded
   Dim JSONGenerator As JSONGenerator

End Sub

The end result is this:

    "menu": {
      "id": "file",
      "value": "File",
      "popup": {
        "menuitem": [
            "value": "New",
            "onclick": "CreateNewDoc()"
            "value": "Open",
            "onclick": "OpenDoc()"
            "value": "Close",
            "onclick": "CloseDoc()"

AJAX Calls
In many cases, when you call a REST API you receive a JSON string. We can use what we’ve learned in the previous topic to retrieve the data we want.

Making an AJAX call (to your own B4J server or an external one) can be done like this. The example is getting a test json file from JSONPlaceholder, a Fake Online REST API for testing and protoyping (https://jsonplaceholder.typicode.com).

Note: because I’m running this test locally, I use ‘jsonp’ as the dataType otherwise Chrome will not allow a cross-domain call.

Page.B4JSCallAjax("myJob1", "https://jsonplaceholder.typicode.com/posts/1", "GET", "jsonp", "" , "B4JSCalculateDistance")

the first parameter is the ‘jobID’ (somewhat like you are used to with JobDone in B4X)
The last parameter is where you want to get the events back (an empty string returns to the ABM Page class, else to the B4JS class name specified)

In the ABM Page class we can do something similar, but the events are always returned to the server (ABM Page class)

Page.CallAjax("myJob1", "https://jsonplaceholder.typicode.com/posts/1", "GET", "jsonp", "")

There are couple events we can use:

Note: in this example I’m using the B4JS versions:

Page_AjaxResult and Page_B4JSAjaxResult

Sub Page_B4JSAjaxResult(uniqueId As String, result As String) As Boolean
   Log("Result from: " & uniqueId & " = " & result)
   Dim JSON As JSONParser
   Dim Map1 As Map
   Map1 = JSON.NextObject
   Log("UsedID: " & Map1.Get("userId"))
   ' if True, it will not call the Page_ajaxResult declared in the Page class!
   Return True
End Sub

Page_AjaxError and Page_B4JSAjaxError

Sub Page_B4JSAjaxError(uniqueId As String, error As String)
   Log("We got an error: " & error)
End Sub

The result is what we would expect:

Result from: myJob1 = {"userId":1,"id":1,"title":"sunt aut facere repellat provident occaecati excepturi optio reprehenderit","body":"quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"}
UsedID: 1

Notice our ‘myJob1’ identifier we used in the CallAjax method.

This concludes the basic B4X to B4JS tools. In the next tutorial(s) we’ll dive into how to use this together with ABMaterial.


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