Add Ptr#eval

Author: ucarion

src/index.test.ts

@@ -1,4 +1,4 @@
-import Ptr, { InvalidPtrError } from "./index";
+import Ptr, { InvalidPtrError, EvalError } from "./index";
 
 describe("Ptr", () => {
   describe("parse", () => {
@@ -35,4 +35,24 @@ describe("Ptr", () => {
       }).toThrowError(new InvalidPtrError(" "));
     });
   });
+
+  describe("eval", () => {
+    it("handles evaluating JSON pointers against any input", () => {
+      expect(Ptr.parse("").eval(null)).toEqual(null);
+      expect(Ptr.parse("").eval(true)).toEqual(true);
+      expect(Ptr.parse("").eval(3.14)).toEqual(3.14);
+      expect(Ptr.parse("").eval("foo")).toEqual("foo");
+      expect(Ptr.parse("").eval([])).toEqual([]);
+      expect(Ptr.parse("").eval({})).toEqual({});
+      expect(Ptr.parse("/foo").eval({ foo: "bar" })).toEqual("bar");
+      expect(Ptr.parse("/0").eval(["bar"])).toEqual("bar");
+      expect(Ptr.parse("/foo/1/bar").eval({foo: [null, { bar: "x" }]})).toEqual("x");
+    });
+
+    it("returns an error when an instance lacks a property", () => {
+      expect(() => { Ptr.parse("/foo").eval(3.14) }).toThrow(new EvalError(3.14, "foo"));
+      expect(() => { Ptr.parse("/0").eval([]) }).toThrow(new EvalError([], "0"));
+      expect(() => { Ptr.parse("/foo").eval({}) }).toThrow(new EvalError({}, "foo"));
+    });
+  });
 });

src/index.ts

@@ -37,6 +37,18 @@ export default class Ptr {
 
     return `/${tokens.join("/")}`;
   }
+
+  public eval(instance: any): any {
+    for (const token of this.tokens) {
+      if (instance.hasOwnProperty(token)) {
+        instance = instance[token];
+      } else {
+        throw new EvalError(instance, token);
+      }
+    }
+
+    return instance;
+  }
 }
 
 export class InvalidPtrError extends Error {
@@ -47,3 +59,14 @@ export class InvalidPtrError extends Error {
     this.ptr = ptr;
   }
 }
+
+export class EvalError extends Error {
+  public instance: any;
+  public token: string;
+
+  constructor(instance: any, token: string) {
+    super(`Error evaluating JSON Pointer: no attribute ${token} on ${instance}`);
+    this.instance = instance;
+    this.token = token;
+  }
+}