feat: create b58 string on creation and throw on id mutation

src/index.js

@@ -18,7 +18,7 @@ class PeerId {
     }
 
     this._id = id
-    this._idB58String = ''
+    this._idB58String = mh.toB58String(this.id)
     this._privKey = privKey
     this._pubKey = pubKey
   }
@@ -27,6 +27,10 @@ class PeerId {
     return this._id
   }
 
+  set id (val) {
+    throw new Error('Id is immutable')
+  }
+
   get privKey () {
     return this._privKey
   }
@@ -82,10 +86,6 @@ class PeerId {
   }
 
   toB58String () {
-    if (!this._idB58String) {
-      this._idB58String = mh.toB58String(this.id)
-    }
-
     return this._idB58String
   }
 }

test/index.spec.js

@@ -29,6 +29,17 @@ describe('PeerId', () => {
     })
   })
 
+  it('throws on changing the id', (done) => {
+    PeerId.create((err, id) => {
+      expect(err).to.not.exist
+      expect(id.toB58String().length).to.equal(46)
+      expect(() => {
+        id.id = new Buffer('hello')
+      }).to.throw(/immutable/)
+      done()
+    })
+  })
+
   it('recreate an Id from Hex string', () => {
     const id = PeerId.createFromHexString(testIdHex)
     expect(testIdBytes).to.deep.equal(id.id)