javascript prepend element to array

var a = [1, 2, 3, 4];
a.unshift(0);
a; // => [0, 1, 2, 3, 4]
// Build an array of test data.
var data = [ "X" ];

// Unshift data onto the array. Unshift() prepends elements to
// the beginning of the given array. Note that it can take more
// than one argument. In the output, notice that when unshift()
// takes multiple arguments, they are prepended in a right-to-left
// order (mimicing their appearence in the arguments list).
data.unshift( "A" );
data.unshift( "B", "C" );

// Output resultant array.
console.log( data );