this_will_work.js

// Defining this variable before the function you want to be run once is executed
// Also setting the value to something other than what you're checking it for
var thisHasAlreadyRun = false

var someFunction = function() {
  // First time through, the variable is defined and the value isn't true, 
  // so it skips the short-circuit return
  if (thisHasAlreadyRun === true) {
    return
  }
  
  // NOW set the flag to true
  thisHasAlreadyRun = true
  // and do the other shit
  doSomethingYouOnlyWantDoneOnce()
}

someFunction()

this_wont_work.js

var someFunction = function() {
  // thisHasAlreadyRun is not defined with "var" (or "let" in ES6) anywhere,
  // so checking its value will always cause an exception
  if (thisHasAlreadyRun === true) {
    return
  }
  
  thisHasAlreadyRun = true
  doSomethingYouOnlyWantDoneOnce()
}

someFunction()