WasmThemis utilities (#489)

* Correctly report error codes

Classical off-by-one mistake. We incorrectly reported error codes as
'undefined' when doing "new ThemisError(subsystem, errorCode)". This
affects only human-readable description, not the 'errorCode' property,
but it's very annoying for developers.

Use ThemisError.describe only when there are 3 (three) or more arguments
with human-readable error causes.

* Move 'coerceToBytes' into utility module

This function is going to be useful in multiple places for type checking
of user byte arrays. Let's move it into a common utility module. While
we're here, let it throw 'TypeError' instead of 'ThemisError' which
should be more recognizable (and does not expect Themis "subsystem").

src/wrappers/themis/wasm/src/secure_keygen.js

@@ -19,6 +19,7 @@
 
 const libthemis = require('./libthemis.js')
 const errors = require('./themis_error.js')
+const utils = require('./utils.js')
 
 const subsystem = 'KeyPair'
 
@@ -40,31 +41,20 @@ const keyKinds = {
 
 class PrivateKey extends Uint8Array {
     constructor(array) {
-        array = coerceToBytes(array)
+        array = utils.coerceToBytes(array)
         super(array)
         validateKeyBuffer(this, [keyKinds.EC_PRIVATE, keyKinds.RSA_PRIVATE])
     }
 }
 
 class PublicKey extends Uint8Array {
     constructor(array) {
-        array = coerceToBytes(array)
+        array = utils.coerceToBytes(array)
         super(array)
         validateKeyBuffer(this, [keyKinds.EC_PUBLIC, keyKinds.RSA_PUBLIC])
     }
 }
 
-function coerceToBytes(key) {
-    if (key instanceof Uint8Array) {
-        return key
-    }
-    if (key instanceof ArrayBuffer) {
-        return new Uint8Array(key)
-    }
-    throw new ThemisError(subsystem, ThemisErrorCode.INVALID_PARAMETER,
-        'key type mismatch, expect "Uint8Array" or "ArrayBuffer"')
-}
-
 function validateKeyBuffer(buffer, expectedKinds) {
     if (buffer.length == 0) {
         throw new ThemisError(subsystem, ThemisErrorCode.INVALID_PARAMETER,

src/wrappers/themis/wasm/src/themis_error.js

@@ -47,7 +47,7 @@ const errorCodes = {
 class ThemisError extends Error {
     constructor(subsystem, errorCode) {
         var message = 'Themis: ' + subsystem + ': '
-        if (arguments.length >= 2) {
+        if (arguments.length > 2) {
             message += arguments[2]
             for (var i = 3; i < arguments.length; i++) {
                 message += ', '

src/wrappers/themis/wasm/src/utils.js

@@ -0,0 +1,33 @@
+// Copyright (c) 2019 Cossack Labs Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * @file
+ * Miscellaneous utilities.
+ */
+
+/**
+ * Convert an object into a byte buffer.
+ *
+ * @throws TypeError if input type is not supported
+ */
+module.exports.coerceToBytes = function(buffer) {
+    if (buffer instanceof Uint8Array) {
+        return buffer
+    }
+    if (buffer instanceof ArrayBuffer) {
+        return new Uint8Array(buffer)
+    }
+    throw new TypeError('type mismatch, expect "Uint8Array" or "ArrayBuffer"')
+}