Now, let's understand the complete code first. The comments explain each relevant step within the code:

function suggestZipCodes() {
  //defined possible ZipCode
  ZipCodes = [
    { name: '94102' },
    { name: '94103' },
    { name: '94104' },
    { name: '94105' },
    { name: '94107' },
    { name: '94108' }
  ];
  var OnZipCodekeyPress = function (fld) {
    var ZipCodetxt = Xrm.Page.getControl
    ("address1_postalcode").getValue();
    resultSet = {
      results: new Array(),
      commands: {
        id: "ZipCodecmd",
        label: "Search in Bing",
        action: function () {
          window.open("http://bing.com"); //Open Bing URL
        }
      }
    };
    var ZipCodetxtLowerCase = ZipCodetxt.toLowerCase();
    for (i = 0; i < ZipCodes.length; i++) {
      if (ZipCodetxtLowerCase ===
      ZipCodes[i].name.substring(0, 
      ZipCodetxtLowerCase.length).toLowerCase()) {
        resultSet.results.push({
        id: i,
        fields: [ZipCodes[i].name]
        });
      }
      if (resultSet.results.length >= 10) {
        break;
      }
    }
    if (resultSet.results.length > 0) {
      //Show Auto Complete
      fld.getEventSource().showAutoComplete(resultSet);
    }
    else {
      //Hide Auto Complete
      fld.getEventSource().hideAutoComplete();
    }
  };
  Xrm.Page.getControl("address1_postalcode").addOnKeyPress
  (OnZipCodekeyPress);
}

Now, let's look at the steps to be performed after completing the code:

1. Navigate to Settings | Customization | Customize the System and select Web Resources. Click New:

2. Name the web resource as zipCodeAutoComplete. Select Type as Script(JScript) and paste the preceding code in the Text Editor link:

3. On clicking the Text Editor button, you will be able to paste the code into it:

4. Click OK. On the main screen, click Save and Publish. Next, navigate to Contact entity | Forms. Open the Contact main form by double-clicking on it:

5. Click on the Form Properties ribbon button on the form editor:

6. Attach the WebResource script (created earlier to form libraries) by clicking Form Properties and attach the method name to the On Load event handler:

7. Click OK. Then click on Save and Publish to publish the form.
8. Now, as soon as any employee types in to the ZIP code field, he or she is presented with options for assistance, diminishing the chances of error:

This is how we can get autocomplete selection for ZIP codes in our forms.