Is it possible to evaluating object/array accessors in JavaScript without using eval()?

gistfile1.js

/*
Scenario: App requests the user to provide accessors to an JSON object that they also provide.
e.g. User provides the following JSON object:
  { people: [ 
     { firstName: "Bob", lastName: "Jones" },
     { firstName: "Jane", lastName: "Jones" } 
    ], numbers: [1, 2, 3]
  }

The JSON object is saved to the 'data' variable by the app.

Then we request that the user specify what element they want to access and ask for a string input.
e.g. The user types '.people[0].firstName', expecting the output to be 'Bob'

We then save the user input into the 'userInput' variable as a string.
*/

eval('data' + userInput); // This will obviously work, but is it possible without using eval()?