Why is my code in the reduce function not working?

When the following JavaScript code is executed, I want the code "action.type" or "action.params" to work.

actions.slice().parallel().reduce((acc, rawAction) => {
  const actionType = rawAction[0]
  const actionParams = rawAction.slice(1)
  const action = {type : actionType, params: actionParams, }
  return acc.concat([action, ])
}, []).concat()

But, after the function is executed, when I am debugging, "action.type" and "action.params" is not defined. (undefined message)

It looks like a simple String array rather than an action Object consisting of the keys "type" and "params".

Why does it work and what should I do?

Thank you.