Add preamble function for NSJS programs.

package.json

@@ -32,6 +32,7 @@
     "supports-color": "^4.2.1",
     "tapable": "^0.2.7",
     "uglifyjs-webpack-plugin": "^0.4.6",
+    "uuid": "^3.2.1",
     "w3c-blob": "0.0.1",
     "watchpack": "^1.4.0",
     "webpack-sources": "^1.0.1",

src/NetscriptJSPreamble.js

@@ -0,0 +1,42 @@
+// A utility function that adds a preamble to each Netscript JS
+// script. This preamble will set all the global functions and
+// variables appropriately for the module.
+//
+// One caveat is that we don't allow the variables in the preable
+// to change. Unlike in normal Javascript, this would not change
+// properties of self. It would instead just change the variable
+// within the given module -- not good! Users should not really
+// need to do this anyway.
+
+import uuidv4 from "uuid/v4";
+import {sprintf} from "sprintf-js";
+
+window.__NSJS__environments = {};
+
+// Returns the UUID for the env.
+export function registerEnv(env) {
+    const uuid = uuidv4();
+    window.__NSJS__environments[uuid] = env;
+    return uuid;
+}
+
+export function unregisterEnv(uuid) {
+    delete window.__NSJS__environments[uuid];
+}
+
+export function makeEnvHeader(uuid) {
+    if (!(uuid in window.__NSJS__environments)) throw new Error("uuid is not in the environment" + uuid);
+
+    const env = window.__NSJS__environments[uuid];
+    var envLines = [];
+    for (const prop in env) {
+        envLines.push("const ", prop, " = ", "__NSJS_ENV[\"", prop, "\"];\n");
+    }
+
+    return sprintf(`
+        'use strict';
+        const __NSJS_ENV = window.__NSJS__environments['%s'];
+        // The global variable assignments (hack, weaken, etc.).
+        %s
+    `, uuid, envLines.join(""));
+}