In Splunk Synthetic, how do you reference a custom variable in your down stream javaScript step

[Jerry07]

In the Splunk Synthetic doc, it mentioned, you need to use the following syntax to reference the custom variable you defined in earlier on steps. 

{{custom.$variable}}

However, when I tried, it seems that syntax only works when you try to reference it in the "Fill in Field" sort of steps.

If I have a downstream javaScript step, and in that javaScript,  I want to reference the custom variable I defined in my earlier on steps,  how do I reference them?

I tried the  {{custom.$variable}} syntax, they do NOT work in my javaScript