How can I use JavaScript within an Excel macro?

The simplest approach may be to embed the Javascript diff logic into a COM component directly using Javascript. This is possible via something called "Windows Script Components".

Here's a tutorial on creating WSCs.

A Windows Script Component is a COM component that is defined in script. The interface to the component is via COM, which means it is VBA friendly. The logic is implemented in any Windows Scripting Hosting -compatible language, like JavaScript or VBScript. The WSC is defined in a single XML file, which embeds the logic, the component Class ID, the methods, the registration logic, and so on.

There's also a tool available to help in creating a WSC. Basically it is a wizard-type thing that asks you questions and fills in the XML template. Myself, I just started with an example .wsc file and edited it by hand with a text editor. It's pretty self-explanatory.

A COM component defined this way in script (in a .wsc file) is callable just like any other COM component, from any environment that can dance with COM.

UPDATE: I took a few minutes and produced the WSC for GoogleDiff. Here it is.

<?xml version="1.0"?><package><component id="Cheeso.Google.DiffMatchPatch">  <comment>    COM Wrapper on the Diff/Match/Patch logic published by Google at http://code.google.com/p/google-diff-match-patch/.  </comment><?component error="true" debug="true"?><registration  description="WSC Component for Google Diff/Match/Patch"  progid="Cheeso.Google.DiffMatchPatch"  version="1.00"  classid="{36e400d0-32f7-4778-a521-2a5e1dd7d11c}"  remotable="False">  <script language="VBScript">  <![CDATA[    strComponent = "Cheeso's COM wrapper for Google Diff/Match/Patch"    Function Register      MsgBox strComponent & " - registered."    End Function    Function Unregister      MsgBox strComponent & " - unregistered."    End Function  ]]>  </script></registration><public>  <method name="Diff">    <parameter name="text1"/>    <parameter name="text2"/>  </method>  <method name="DiffFast">    <parameter name="text1"/>    <parameter name="text2"/>  </method></public><script language="Javascript"><![CDATA[    // insert original google diff code here...// public methods on the componentvar dpm = new diff_match_patch();function Diff(text1, text2){   return dpm.diff_main(text1, text2, false);}function DiffFast(text1, text2){   return dpm.diff_main(text1, text2, true);}]]></script></component></package>

To use that thing, you have to register it. In Explorer, right click on it, and select "Register". or, from the command line: regsvr32 file:\c:\scripts\GoogleDiff.wsc

I didn't try using it from VBA, but here is some VBScript code that uses the component.

Sub TestDiff()    dim t1     t1 = "The quick brown fox jumped over the lazy dog."    dim t2     t2 = "The large fat elephant jumped over the cowering flea."    WScript.echo("")    WScript.echo("Instantiating a Diff Component ...")    dim d    set d = WScript.CreateObject("Cheeso.Google.DiffMatchPatch")    WScript.echo("Doing the Diff...")    x = d.Diff(t1, t2)    WScript.echo("")    WScript.echo("Result was of type: " & TypeName(x))    ' result is all the diffs, joined by commas.      ' Each diff is an integer (position), and a string.  These are separated by commas.    WScript.echo("Result : " & x)    WScript.echo("Transform result...")    z= Split(x, ",")    WScript.echo("")    redim diffs(ubound(z)/2)    i = 0    j = 0    For Each item in z      If (j = 0) then        diffs(i) = item        j = j+ 1            Else           diffs(i) = diffs(i) & "," & item        i = i + 1        j = 0      End If    Next    WScript.echo("Results:")    For Each item in diffs      WScript.echo("  " & item)    Next    WScript.echo("Done.")End Sub

The Windows Scripting Engine will allow you to run the JavaScript library. It works well in my experience.

My suggestion would be that whatever you do you wrap it in a COM wrapper. VBA deals best with COM objects so you could compile as a .NET Component then expose as a COM object using the interop functionality of .NET.

As an alternative you could also look into using Windows Scripting Host objects to execute a Javascript File and return you the result.