zooming on canvas and allow drawing

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Canvas

I am using this link to achieve zoom in functionality on canvas
http://bl.ocks.org/mbostock/raw/3680958/[^]

From below code i want to replace append("canvas") with the canvas which already exists on page may be by using document.getElementById('canvas1'). After replacing it the node() returns null. What could be the reason?

var canvas = d3.select("body").append("canvas")
  .attr("width", width)
  .attr("height", height)
  .call(d3.behavior.zoom().scaleExtent([1, 8]).on("zoom", zoom))
  .node().getContext("2d");