title: javascript check if file exists
body: <p>i have the following code which uses D3.js to load json data:</p>
  
  <pre><code>this.clickCountry = function(d) {
      "use strict"
      var time_for_remove = 500,
          time_for_zoom = 900
      d3.selectAll("svg g").transition().delay(time_for_remove + time_for_zoom - 200).remove()
      self.svg.append("g")
          .attr("id", "country")
      self.regionsGroup = self.svg.select("#country")
      var adm1_key = d.id+"_adm1"
      try {
          var adm1_path = "../topo/"+d.id+"_adm1.json"
      } catch (e) {
          console.log(e)    // "MyError"
      }
      var adm1_path = "../topo/"+d.id+"_adm1.json"
      d3.json(adm1_path, function(error, topology) {
          var regions = topology.objects
          for(var adm1_key in regions) { 
              var o = regions[adm1_key]
          }
          self.regionsGroup.selectAll("path")
          .data(topojson.object(topology, o).geometries)
          .enter().append("path")
          .attr("d", self.projection)
          .attr("id", function(d) {
              return d.properties.name
          })
          .classed("country", true)
          .attr("class", "country")
          .on("mouseover", function(d) {
              d3.select(this)
              .style("fill", "#6C0")
              .append("svg:title")
              .text(d.properties.name)
          })
          .on("mouseout", function(d) {
              d3.select(this)
              .style("fill", "#000000")
          })
          .on("click", function(d) {
              console.log('clicked on country')
              var p = d3.mouse(this)                                                                     
              console.log(p+" "+self.map.invert(p))                                                          
              self.map.center(self.map.invert(p))
          })
      })
  }
  </code></pre>
  
  <p>what is the correct way to check if the file exists <code>adm1_path = "../topo/"+d.id+"_adm1.json"</code> before actually drawing the map?</p>