Example once implementation

once.js

/** Once takes a function as an argument and returns a function or undefined, depending on whether the fn passed as an argument 
* has been called. If that function hasn't been invoked, Once will return the value of that function applied to any arguments. 
* otherwise it will return undefined. 
*/

// implementation #1
function once (fn) {
  var run = false
  return function () {
    if (run) return void 0
    else {
      run = true
      return fn.apply(this, arguments)
    }
  }
}

// Example
var withdrawCash = once(function () {
  console.log('you withdrew some cash!');
})

withdrawCash() // => 'you withdrew some cash!'
withdrawCash() // => undefined

// implementation #2
const once2 = (fn) => {
  let run = false
  return function () {
    return run 
      ? void 0 
      : ((run = true), fn.apply(this, arguments))
  }
}

// Example
const sayBye = (name) => `bye bye ${name}`
const sayByeOnce = once2(sayBye)

sayByeOnce('Jeremy')    // => 'bye bye Jeremy'
sayByeOnce('Jeremy')   // => undefined
sayByeOnce('Alice')   // => undefined