style: Set up eslint and fix (#16)

All autofixes except for strict mode, added manually.

.eslintignore

@@ -0,0 +1,2 @@
+/coverage
+/node_modules

.eslintrc.yml

@@ -0,0 +1,35 @@
+env:
+  node: true
+
+parserOptions:
+  ecmaVersion: 9
+  # Override eslint-config-standard, which incorrectly sets this to "module",
+  # though that setting is only for ES6 modules, not CommonJS modules.
+  sourceType: 'script'
+
+extends:
+  - 'eslint:recommended'
+  - standard
+  - prettier
+
+rules:
+  # Override some recommended rules.
+  no-unused-vars: ['error', { 'args': 'none' }]
+  no-empty: ['error', { 'allowEmptyCatch': true }]
+
+  # Nock additions.
+  strict: ['error', 'safe']
+  no-loop-func: 'error'
+  no-var: 'error'
+  prefer-const: 'error'
+  object-shorthand: ['error', 'properties']
+  prefer-template: 'error'
+  arrow-body-style: ['error', 'as-needed']
+  prefer-destructuring:
+    [
+      'error',
+      {
+        'VariableDeclarator': { 'array': false, 'object': true },
+        'AssignmentExpression': { 'array': false, 'object': false },
+      },
+    ]

index.js

@@ -1,3 +1,5 @@
+'use strict'
+
 function propagate(events, source, dest) {
   if (arguments.length < 3) {
     dest = source
@@ -6,14 +8,14 @@ function propagate(events, source, dest) {
   }
 
   // events should be an array or object
-  var eventsIsObject = typeof events === 'object'
+  const eventsIsObject = typeof events === 'object'
   if (events && !eventsIsObject) events = [events]
 
   if (eventsIsObject) {
     return explicitPropagate(events, source, dest)
   }
 
-  var oldEmit = source.emit
+  const oldEmit = source.emit
 
   source.emit = function(eventType) {
     oldEmit.apply(source, arguments)
@@ -28,15 +30,15 @@ function propagate(events, source, dest) {
   }
 
   return {
-    end: end,
+    end,
   }
 }
 
 module.exports = propagate
 
 function explicitPropagate(events, source, dest) {
-  var eventsIn
-  var eventsOut
+  let eventsIn
+  let eventsOut
   if (Array.isArray(events)) {
     eventsIn = events
     eventsOut = events
@@ -47,9 +49,9 @@ function explicitPropagate(events, source, dest) {
     })
   }
 
-  var listeners = eventsOut.map(function(event) {
+  const listeners = eventsOut.map(function(event) {
     return function() {
-      var args = Array.prototype.slice.call(arguments)
+      const args = Array.prototype.slice.call(arguments)
       args.unshift(event)
       dest.emit.apply(dest, args)
     }
@@ -58,7 +60,7 @@ function explicitPropagate(events, source, dest) {
   listeners.forEach(register)
 
   return {
-    end: end,
+    end,
   }
 
   function register(listener, i) {