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A Pen By Captain Anonymous

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  Lets say we have an array of names and we want to print them
  to the console joined by a comma.
var names = ["Kwik", "Kwek", "Kwak"];

  This function printArrayJoined comes from some external library. 
  We think this function just prints the names joined by a comma. 

  Unbeknown to us the printArrayJoined actually mutates the array!
  At the end of this function it will be empty.
function printArrayJoined(array) {
  var out = array.join(', ');


// The names array is still what we expect it to be.
console.log(names); // prints ["Kwik", "Kwek", "Kwak"]
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