jasmine-node how to override a variable for testing

I am using jasmine-node to test my node js and I am getting confuse with how to mock some stuff.

let's say this is my code under main.js:

var configFile = "config.json"
function readFile(){
  fs.readFile(configFile, 'utf8', function(err, configData){
      ....
  }
}
export.readFile = readFile

In my test file, I want to override the configFile param with my own configFile for testings

 var main = require("../main.js"); 
 describe("test with my own config file", function(){
    it("check for ...", function(){
        configFile = "otherConfigFile.json"
        main.readFile();    
    } 
 }