Gtk.EntryCompletion.prototype.compute_prefix

function compute_prefix(key:String):String {
    // Gjs wrapper for gtk_entry_completion_compute_prefix()
}

Computes the common prefix that is shared by all rows in completion that start with key. If no row matches key, null will be returned. Note that a text column must have been set for this function to work, see Gtk.set_text_column for details.

Since 3.4