Massage the section more and add details

content/docs/hello-world.md

@@ -29,6 +29,17 @@ The next few sections will gradually introduce you to using React. We will exami
 
 ## Knowledge Level Assumptions
 
-React is a JavaScript library, and so we'll assume you have a basic understanding of the JavaScript language. If you don't feel very confident, we recommend [going through a Javascript tutorial](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript) to check your knowledge level and enable you to follow along this tutorial more easily.
+React is a JavaScript library, and so we'll assume you have a basic understanding of the JavaScript language. **If you don't feel very confident, we recommend [going through a JavaScript tutorial](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript) to check your knowledge level** and enable you to follow along this walkthrough without getting lost. It might take you between 30 minutes and an hour, but as a result you won't have to feel like you're learning both React and JavaScript at the same time.
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+This walkthrough also uses some of the newer JavaScript syntax in the examples. **If you haven't worked with JavaScript in the last few years,** these three points should get you most of the way:
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+* We define variables with [`let`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let) and [`const`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const) statements. For the purposes of this walkthrough, you can consider them equivalent to [`var`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var).
+* We use the `class` keyword to define [JavaScript classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes). There are two things worth remembering about them. Firstly, unlike with objects, you *don't* need to put commas between class method definitions. Secondly, unlike many other languages with classes, in JavaScript the value of `this` in a method [depends on how it is called](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes#Boxing_with_prototype_and_static_methods).
+* We sometimes use `=>` to define ["arrow functions"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions). They're like regular functions, but shorter. For example, `x => x * 2` is roughly equivalent to `function(x) { return x * 2; }`. Importantly, arrow functions [don't have their own `this` value](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#No_separate_this) so they're handy when you want to preserve the `this` value from an outer method definition.
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+Don't worry if this is too much to take in at once. The [MDN JavaScript Reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript) is a stellar resource, and you can consult it whenever you get confused by something.
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+<!-- Note: I intentionally don't use babel:// scheme here because it doesn't seem to include the ES2015 preset which is crucial in this example -->
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+Also, when you feel unsure about what some newer syntax means, you can use the [Babel REPL with the ES2015 preset](http://babeljs.io/repl/#?babili=false&browsers=&build=&builtIns=false&code_lz=MYewdgzgLgBAllApgWwjAvDA2gRgDQwBMBAzALoDcAUKJLACYgCuARgDaL0bxKoB0yAIYAHABQAPDAD4YkgFREAlBSA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&lineWrap=true&presets=es2015%2Creact%2Cstage-1%2Cstage-2%2Cstage-3&prettier=true&targets=Node-6.12&version=6.26.0&envVersion=) to check what equivalent older syntax it compiles to.
 
-The tutorial also includes some of the ES6 syntax in the examples. We have try to use it sparingly because it's still relatively new, but we encourage you to get familiar with [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions), [classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), [template literals](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Template_literals), [`let`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let), and [`const`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const) statements. You can use the [Babel REPL](babel://es5-syntax-example) to check what ES6 code compiles to.