var fs = require('fs');

var environmentVariables = [
    "WEBSITE_SITE_NAME",
    "WEBSITE_SKU",
    "WEBSITE_COMPUTE_MODE",
    "WEBSITE_HOSTNAME",
    "WEBSITE_INSTANCE_ID",
    "WEBSITE_NODE_DEFAULT_VERSION",
    "WEBSOCKET_CONCURRENT_REQUEST_LIMIT",
    "APPDATA",
    "TMP",
    "WEBJOBS_PATH",
    "WEBJOBS_NAME",
    "WEBJOBS_TYPE",
    "WEBJOBS_DATA_PATH",
    "WEBJOBS_RUN_ID",
    "WEBJOBS_SHUTDOWN_FILE"
]

module.exports = function (context, req, res) {
    context.log('function triggered');
    context.log(req);

    context.log('Environment Variables');
    environmentVariables
        .map(getEnvironmentVarible)
        .forEach(function (x) {
            context.log(x);
        });

    context.log('process.cwd()', process.cwd());
    context.log('__dirname', __dirname);

    fs.writeFile('D:/local/Temp/message.txt', 'Hello Node.js', (err) => {
        if (err) throw err;
        context.log("It's saved!");
        context.done();
    });
}

function getEnvironmentVarible(name) {
    return name + ": " + process.env[name];
}
var fs = require('fs');
var path = require('path');

var environmentVariables = [
    "WEBSITE_SITE_NAME",
    "WEBSITE_SKU",
    "WEBSITE_COMPUTE_MODE",
    "WEBSITE_HOSTNAME",
    "WEBSITE_INSTANCE_ID",
    "WEBSITE_NODE_DEFAULT_VERSION",
    "WEBSOCKET_CONCURRENT_REQUEST_LIMIT",
    "APPDATA",
    "TMP",
    "HOME",
    "SYSTEMDRIVE",
    "WEBJOBS_PATH",
    "WEBJOBS_NAME",
    "WEBJOBS_TYPE",
    "WEBJOBS_DATA_PATH",
    "WEBJOBS_RUN_ID",
    "WEBJOBS_SHUTDOWN_FILE",
    "WEBSITE_NODE_DEFAULT_VERSION",
    "WEBSITE_NPM_DEFAULT_VERSION"
]

module.exports = function (context, req, res) {
    context.log('function triggered');
    context.log(req);

    context.log('Environment Variables');
    environmentVariables
        .map(getEnvironmentVarible)
        .forEach(function (x) {
            context.log(x);
        });

    context.log('process.cwd()', process.cwd());
    context.log('__dirname', __dirname);

    var tempDir = process.env["TMP"];
    var filePath = path.join(tmpDir, 'message.txt');
    var fileContents = 'Hellow Node.js_' + req.body;

    context.log("writing: ", fileContents);

    fs.writeFile(filePath, fileContents, (err) => {
        if (err) {
            throw err;
        }

        context.log("File is saved!");
        context.done();
    });
}

function getEnvironmentVarible(name) {
    return name + ": " + process.env[name];
}
Add few more environment variables to test, and use them to construct filepaths for more robustness
Add few more environment variables to test, and use them to construct filepaths for more robustness
