Interpolate custom vars in serverless.yml

interpolateServerless.js

/*
This came about because I needed to get a VPC name that was a function of the deployment stage 
(e.g. 'my-vpc-dev', 'my-vpc-qa', etc.). I could have just hardcoded the VPC name in serverless.yml and edited it for
each deploy, but where's the fun in that?

What this does is to take a value from serverless.yml like "my-vpc-${self:custom.stage}" and interpolate the variable 
"${self:custom.stage}" embedded therein. This variable could resolve to another variable, so it gets called recursively.
Note that the construction "${opt:stage, self:provider.stage}" should be interpreted as 
${opt:stage} || ${self:provider.stage}
*/

const fillVars = (string, serverless) => {
  const sourceMap = {
    self: serverless.variables.service,
    opt: serverless.variables.options,
  }
  const match = string.match(/\${.+?}/g)
  if (match) {
    console.debug(`Interpolating "${string}"`)
    match.forEach((matched) => {
      // matched: '${SRC:PATH}'
      const paths = matched.replace(/\${(.+)}/, '$1').split(/,\s*/)
      console.debug({ paths: paths })
      let val = paths.reduce((ret, pathspec) => {
        if (ret) return ret
        const [src, path] = pathspec.split(':')
        console.debug({ src: src, path: path })
        const target = sourceMap[src]
        if (!target) {
          console.warn(`Unsupported variable ${pathspec}`)
        }
        ret = jmespath.search(target, path)
        console.debug(`Value for path "${path}" is "${ret}"`)
        return ret
      }, null)
      //      const [src, path] = matched.match(/\${(\w+):(.+?)}/).slice(1, 3)
      if (val.match(/\${.+?}/)) {
        console.debug(`Recursively interpolating ${val}`)
        val = fillVars(val, serverless)
      }
      string = string.replace(matched, val)
    })
  }
  return string
}