getElementsByClassName

I am trying to select same elements with different parents that have the same class using getElementsByClassName, but i get the following error message:

org.mozilla.javascript.EcmaError: TypeError: Cannot find function getElementsByClassName in object com.arbortext.epic.internal.aom.DocumentImpl@156d16c0. (C:\program files\ptc\arbortext editor\custom\scripts\Local.js#3985)

    at org.mozilla.javascript.ScriptRuntime.constructError(ScriptRuntime.java:3689)

    at org.mozilla.javascript.ScriptRuntime.constructError(ScriptRuntime.java:3667)

    at org.mozilla.javascript.ScriptRuntime.typeError(ScriptRuntime.java:3695)

    at org.mozilla.javascript.ScriptRuntime.typeError2(ScriptRuntime.java:3714)

    at org.mozilla.javascript.ScriptRuntime.notFunctionError(ScriptRuntime.java:3786)

    at org.mozilla.javascript.ScriptRuntime.getPropFunctionAndThisHelper(ScriptRuntime.java:2268)

    at org.mozilla.javascript.ScriptRuntime.getPropFunctionAndThis(ScriptRuntime.java:2250)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25._c_Renumber_32(C:\program files\ptc\arbortext editor\custom\scripts\Local.js:3985)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25.call(C:\program files\ptc\arbortext editor\custom\scripts\Local.js)

    at org.mozilla.javascript.optimizer.OptRuntime.callName(OptRuntime.java:63)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25._c_InsertStep_14(C:\program files\ptc\arbortext editor\custom\scripts\Local.js:1228)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25.call(C:\program files\ptc\arbortext editor\custom\scripts\Local.js)

    at org.mozilla.javascript.optimizer.OptRuntime.callName(OptRuntime.java:63)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25._c_InsertElements_11(C:\program files\ptc\arbortext editor\custom\scripts\Local.js:316)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25.call(C:\program files\ptc\arbortext editor\custom\scripts\Local.js)

    at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:394)

    at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3090)

    at org.mozilla.javascript.gen.C__program_files_ptc_arbortext_editor_custom_scripts_Local_js_25.call(C:\program files\ptc\arbortext editor\custom\scripts\Local.js)

    at org.mozilla.javascript.JavaAdapter.doCall(JavaAdapter.java:606)

    at org.mozilla.javascript.JavaAdapter.access$000(JavaAdapter.java:15)

    at org.mozilla.javascript.JavaAdapter$1.run(JavaAdapter.java:586)

    at org.mozilla.javascript.Context.call(Context.java:489)

    at org.mozilla.javascript.ContextFactory.call(ContextFactory.java:504)

    at org.mozilla.javascript.JavaAdapter.callMethod(JavaAdapter.java:583)

    at adapter2.handleEvent(<adapter>)

Any idea why getElementsByClassName cannot by found? How would I get getElementsByClassName to work?

Thank you for help!

Have a blessed day!

Florin