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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 = '';

  var length = array.length;
  while(length !== 1) {
    out += array.shift() + ", ";

    length -= 1;

  out += array.shift();


printArrayJoined(names); // prints "Kwik, Kwek, Kwak"

// At this point the names array is empty. Not what we expected.
console.log(names); // prints []
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