Read 2 json files.

script.js

// file読み込み制御用
var readTopofileDone = false;
var readAttrfileDone = false;
// filedata格納変数
var topodata;
var attrdata;

var map_width = 500;
var map_height = 650;
var svg;

function readTopofile(json) {
    topodata = json;
    readTopofileDone = true;
}
function readAttrfile(json) {
    attrdata = json;
    readAttrfileDone = true;
}

d3.json("yokohama_topo_out.json", function(error, json) {
    if(error) return console.warn(error);
        readTopofile(json);
        if (readTopofileDone && readAttrfileDone) {
            main(topodata, attrdata);
        }
    });

d3.json("yokohama_codes.json", function(error, json) {
    if(error) return console.warn(error);
        readAttrfile(json);
        if (readTopofileDone && readAttrfileDone) {
            main(topodata, attrdata);
        }
    });

function main(topo, attr) {
    // break
}