dist/vue.runtime.common.js
function once (fn) {
  var called = false;
  return function () {
    if (!called) {
      called = true;
      fn.apply(this, arguments);
    }
  }
}
function once (fn) {
  var called = false;
  return function () {
    if (!called) {
      called = true;
      fn.apply(this, arguments);
    }
  }
}
function once ( fn ) { var called = false ; return function ( ) { if ( ! called ) { called = true ; fn . apply ( this , arguments ) ; } } }
Ensure a function is called only once.
Ensure a function is called only once .