The shift() method shifts off and returns the first element of the array, decreasing its size by one. Here is the method to be declared as a prototype of the Array object type:

function shift(str) {
  var val = this[0];
  for (var i = 1; i < this.length; ++i) {
    this[i-1] = this[i];
  }
  this.length--;
  return val;
}

The first line sets the returned value, the first element of the array. The for loop iterates over the array elements and shifts every element to its previous position:

this[i-1] = this[i];

The last two lines decrement the size of the array by one and returned the value of the first element, val.

When the browser is a pre-5.5 version of Internet Explorer, we need to extend the Array's prototype with the new shift() method:

if (bName() == 1 && bVer() >= 5.5)
else {
  Array.prototype.shift = shift;
}

We do nothing when the browser is IE 5.5 or higher. For other browsers or lower versions, we assign the home-brewed shift() method.

The unshift() method appends a list of elements to the beginning of the array. Here is our implementation:

function unshift() {
  var i = unshift.arguments.length;
  for (var j = this.length - 1; j >= 0; --j) {
    this[j + i] = this[j];
  }
  for (j = 0; j < i; ++j) {
    this[j] = unshift.argument[j];
  }
}

We extract the number of new elements, i, from the arguments array's length. The first for loop shifts all existing elements i places up:

this[j + i] = this[j];

The second for loop inserts the new elements into the first i places of the array:

this[j] = unshift.arguments[j];

When the browser is a pre-5.5 version, we need to extend the Array's prototype with the new unshift() method:

if (bName() == 1 && bVer() >= 5.5)
else {
  Array.prototype.unshift = unshift;
}

We do nothing when the browser is IE 5.5 or higher. For lower versions, we assign the home-brewed unshift() method.

See a demo of the above methods in Column 59, IE 5.5: Formatting, URIs, and Stack Operations.

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