add missing src/guard.mjs from template

src/guard.mjs

@@ -0,0 +1,72 @@
+/**
+ * Enforces conditions of routes
+ * Like the presents of values in the context
+ */
+export class Guard {
+  /**
+   * Called while entering a route (current outlet is not yet set)
+   * @param {Transition} transition
+   */
+  async enter(transition) {}
+
+  /**
+   * Called before leaving a route
+   * @param {Transition} transition
+   */
+  async leave(transition) {}
+
+  toString() {
+    return this.constructor.name;
+  }
+}
+
+/**
+ * Redirects to a given path if condition is met.
+ *
+ * @param {string} path
+ * @param {Function} condition redirects when returning true
+ */
+export function redirectGuard(path, condition) {
+  return {
+    toString: () => `redirect(${path})`,
+    enter: async transition => {
+      if (condition === undefined || condition(transition)) {
+        return transition.redirect(path);
+      }
+    },
+    leave: () => {}
+  };
+}
+
+/**
+ * Execute guards in a sequence.
+ * @param {Iterable<Guard>} children
+ */
+export function sequenceGuard(children) {
+  return {
+    toString: () => children.toString(),
+    enter: async transition => {
+      for (const child of children) {
+        await child.enter(transition);
+      }
+    },
+    leave: async transition => {
+      for (const child of children) {
+        await child.leave(transition);
+      }
+    }
+  };
+}
+
+/**
+ * Execute guards in a parallel.
+ * @param {Iterable<Guard>} children
+ */
+export function parallelGuard(children) {
+  return {
+    enter: async transition =>
+      Promise.all([...children].map(c => c.enter(transition))),
+    leave: async transition =>
+      Promise.all([...children].map(c => c.leave(transition)))
+  };
+}