Add eslint config, and fix problems reported

.eslintrc

+{
+  "env": {
+    "browser": true,
+    "amd": true
+  },
+  "globals": {
+    "M": true,
+    "Y": true,
+    "H5PIntegration": true,
+    "H5PEditor": true,
+    "H5P": true
+  },
+  "rules": {
+    "comma-dangle": "off",
+    "no-compare-neg-zero": "error",
+    "no-cond-assign": "error",
+    "no-console": ["error", {"allow": ["warn", "error"]}],
+    "no-constant-condition": "error",
+    "no-control-regex": "error",
+    "no-debugger": "error",
+    "no-dupe-args": "error",
+    "no-dupe-keys": "error",
+    "no-duplicate-case": "error",
+    "no-empty": "warn",
+    "no-empty-character-class": "error",
+    "no-ex-assign": "error",
+    "no-extra-boolean-cast": "error",
+    "no-extra-parens": "off",
+    "no-extra-semi": "error",
+    "no-func-assign": "error",
+    "no-inner-declarations": "error",
+    "no-invalid-regexp": "error",
+    "no-irregular-whitespace": "error",
+    "no-obj-calls": "error",
+    "no-prototype-builtins": "off",
+    "no-regex-spaces": "error",
+    "no-sparse-arrays": "error",
+    "no-unexpected-multiline": "error",
+    "no-unreachable": "warn",
+    "no-unsafe-finally": "error",
+    "no-unsafe-negation": "error",
+    "use-isnan": "error",
+    "valid-jsdoc": ["warn", { "requireReturn": false, "requireParamDescription": false, "requireReturnDescription": false }],
+    "valid-typeof": "error",
+    "array-callback-return": "warn",
+    "block-scoped-var": "warn",
+    "complexity": "warn",
+    "consistent-return": "warn",
+    "curly": "error",
+    "dot-notation": "warn",
+    "no-alert": "warn",
+    "no-caller": "error",
+    "no-case-declarations": "error",
+    "no-div-regex": "error",
+    "no-empty-pattern": "error",
+    "no-empty-function": "warn",
+    "no-eq-null": "error",
+    "no-eval": "error",
+    "no-extend-native": "error",
+    "no-extra-bind": "warn",
+    "no-fallthrough": "error",
+    "no-floating-decimal": "warn",
+    "no-global-assign": "warn",
+    "no-implied-eval": "error",
+    "no-invalid-this": "error",
+    "no-iterator": "error",
+    "no-labels": "error",
+    "no-loop-func": "error",
+    "no-multi-spaces": "warn",
+    "no-multi-str": "error",
+    "no-new-func": "error",
+    "no-new-wrappers": "error",
+    "no-octal": "error",
+    "no-octal-escape": "error",
+    "no-proto": "error",
+    "no-redeclare": "warn",
+    "no-return-assign": "error",
+    "no-script-url": "error",
+    "no-self-assign": "error",
+    "no-self-compare": "error",
+    "no-sequences": "warn",
+    "no-throw-literal": "warn",
+    "no-unmodified-loop-condition": "error",
+    "no-unused-expressions": "error",
+    "no-unused-labels": "error",
+    "no-useless-call": "warn",
+    "no-useless-escape": "warn",
+    "no-with": "error",
+    "wrap-iife": ["error", "any"],
+
+    "no-delete-var": "error",
+    "no-undef": "error",
+    "no-undef-init": "error",
+    "no-unused-vars": ["error", { "caughtErrors": "none" }],
+
+    "array-bracket-spacing": "warn",
+    "block-spacing": "warn",
+    "brace-style": ["warn", "1tbs"],
+    "camelcase": "warn",
+    "capitalized-comments": ["warn", "always", { "ignoreConsecutiveComments": true }],
+    "comma-spacing": ["warn", { "before": false, "after": true }],
+    "comma-style": ["warn", "last"],
+    "computed-property-spacing": "error",
+    "consistent-this": "off",
+    "eol-last": "off",
+    "func-call-spacing": ["warn", "never"],
+    "func-names": "off",
+    "func-style": "off",
+    "indent": ["off", 4, { "SwitchCase": 1 }],
+    "key-spacing": ["warn", { "beforeColon": false, "afterColon": true, "mode": minimum }],
+    "keyword-spacing": "warn",
+    "linebreak-style": ["error", "unix"],
+    "lines-around-comment": "off",
+    "max-len": ["error", 132],
+    "max-lines": "off",
+    "max-depth": "warn",
+    "max-nested-callbacks": ["warn", 5],
+    "max-params": "off",
+    "max-statements": "off",
+    "max-statements-per-line": ["warn", { max: 2 }],
+    "new-cap": ["warn", { "properties": false }],
+    "new-parens": "warn",
+    "newline-after-var": "off",
+    "newline-before-return": "off",
+    "newline-per-chained-call": "off",
+    "no-array-constructor": "off",
+    "no-bitwise": "error",
+    "no-continue": "off",
+    "no-inline-comments": "off",
+    "no-lonely-if": "off",
+    "no-mixed-operators": "off",
+    "no-mixed-spaces-and-tabs": "error",
+    "no-multiple-empty-lines": "warn",
+    "no-negated-condition": "off",
+    "no-nested-ternary": "warn",
+    "no-new-object": "off",
+    "no-plusplus": "off",
+    "no-tabs": "error",
+    "no-ternary": "off",
+    "no-trailing-spaces": "error",
+    "no-underscore-dangle": "off",
+    "no-unneeded-ternary": "off",
+    "no-whitespace-before-property": "warn",
+    "object-curly-newline": "off",
+    "object-curly-spacing": "warn",
+    "object-property-newline": "off",
+    "one-var": "off",
+    "one-var-declaration-per-line": ["warn", "initializations"],
+    "operator-assignment": "off",
+    "operator-linebreak": "off",
+    "padded-blocks": "off",
+    "quote-props": ["warn", "as-needed", {"unnecessary": false, "keywords": true, "numbers": true}],
+    "quotes": "off",
+    "require-jsdoc": "warn",
+    "semi": "error",
+    "semi-spacing": ["warn", {"before": false, "after": true}],
+    "sort-vars": "off",
+    "space-before-blocks": "warn",
+    "space-before-function-paren": ["warn", "never"],
+    "space-in-parens": "warn",
+    "space-infix-ops": "warn",
+    "space-unary-ops": "warn",
+    "spaced-comment": "warn",
+    "unicode-bom": "error",
+    "wrap-regex": "off",
+
+    "no-restricted-properties": ["warn", {
+        "object": "M",
+        "property": "str",
+        "message": "Use AMD module 'core/str' or M.util.get_string()"
+    }],
+
+  },
+  overrides: [
+    {
+      files: ["**/yui/src/**/*.js"],
+
+      rules: {
+        "no-undef": "off",
+        "no-unused-vars": "off",
+        "no-unused-expressions": "off"
+      }
+    },
+    {
+      files: ["**/amd/src/*.js"],
+      rules: {
+        "no-unused-vars": "error",
+        "no-implicit-globals": "error"
+      }
+    }
+  ]
+}

dataviews.js

-/*global H5PDataView H5PIntegration H5P*/
+/* global H5PDataView */
 (function($) {
 
     /**

editor.js

-/*global H5PIntegration H5PEditor H5P*/
 (function($) {
 
   /**
    * Get closest row of element
    *
-     * @param $el
-     * @returns jQuery
+   * @param {jQuery} $el
+   * @returns {jQuery}
    */
   function getRow($el) {
     return $el.closest('.fitem');

embed.js

@@ -129,8 +129,7 @@ document.onreadystatechange = function (){
                         clientHeight: iFrame.contentDocument.body.clientHeight
                     }
                 );
-            }
-            else {
+            } else {
                 H5PEmbedCommunicator.send('hello');
             }
         }, 0);

settings-hide-key.js

-/*global HVPSettingsHideKey H5P*/
+/* global HVPSettingsHideKey */
 /**
  * Prepares for hiding of the Site Key setting since you cannot modify attributes and such in Moodle.
  */

xapi-collector.js

-/*global H5P*/
 /**
  * Collect results from xAPI events
  */
@@ -9,7 +8,7 @@
      *
      * @param  {Array} instances
      * @param  {number} contentId
-     * @returns {Object|null} Content instance
+     * @returns {Object|undefined} Content instance
      */
     function findInstanceInArray(instances, contentId) {
         if (instances !== undefined && contentId !== undefined) {
@@ -19,6 +18,7 @@
                 }
             }
         }
+        return undefined;
     }
 
     /**
@@ -39,8 +39,7 @@
                 // Assume first iframe.
                 instance = iframes[0].contentWindow.H5P.instances[0];
             }
-        }
-        else {
+        } else {
             // Try this documents instances.
             instance = findInstanceInArray(H5P.instances, contentId);
             if (!instance) {
@@ -72,8 +71,7 @@
         // Use getXAPIData contract, needed to get children.
         if (instance && instance.getXAPIData) {
             xAPIData = instance.getXAPIData();
-        }
-        else {
+        } else {
             // Fallback to event data.
             xAPIData = {
                 statement: event.data.statement
@@ -86,7 +84,7 @@
             xAPIResult: JSON.stringify(xAPIData)
         }).done(function(data) {
             if (data.error) {
-                console.debug('Storing xAPI results failed with error message:', data);
+                console.error('Storing xAPI results failed with error message:', data);
             }
         });
     }
@@ -94,13 +92,13 @@
     $(document).ready(function() {
         // No external dispatcher.
         if (!(window.H5P && H5P.externalDispatcher)) {
-            console.debug('External dispatcher not found');
+            console.error('External dispatcher not found');
             return;
         }
 
         // No ajax path.
         if (!(window.H5PIntegration && H5PIntegration.ajax && H5PIntegration.ajax.xAPIResult)) {
-            console.debug('No ajax path found');
+            console.error('No ajax path found');
             return;
         }