Quick notes on advanced ES2015 features

Jan 7, 2016

[Note: I found this blog post 5 months after I first wrote it. It is not finished to the standard that I originally planned but I decided I'd still rather publish it now than delete it.]

I first started programming JavaScript with vanilla ES5. I disliked it right from the start having grown accustomed to the nice syntax of Python and ended up switching to CoffeeScript for quite a while. However, around June of last year, I got interested in ES6/ES2015 again.

These are some of my notes from reading this book on ES6 features. I deliberately didn't write about anything I already found trivial, deciding to only focus on things I didn't already know. I never got to the end of the book but still decided to publish it.

New methods

  • String includes(), startsWith() and endsWith(), repeat()
  • Using let or const in for loops creates a new copy of each variable instead of re-using the same one
  • Nested destructuring var { repeat, save, rules: { custom }} = options;
  • If not using let or const or var for destructuring then must use brackets ({ repeat, save, rules: { custom }} = options);
  • Mixed destructuring var { repeat, save, colors: [ firstColor, secondColor ]} = options;
  • Number.isFinite() and Number.isNaN() (not the same as the globally scoped versions because they don't accept strings
  • Number.isSafeInteger() to check if a number is within the range where integer rounding errors won't occur
  • New Math methods: hypot, log2, log10, sign, trunc
  • Object.is() for a better version of ===

  • Default arguments used when argument is not passed OR is passed as undefined (BUT NOT null)

  • Default arguments are re-evaluated on each invocation (which is DIFFERENT to how Python does it)
  • Default arguments for destructuring parameters function setCookie(name, value, { secure, path, domain, expires } = {}) {

  • All functions have a "name" property

function doSomething() {}
var doAnotherThing = function() {};
console.log(doSomething.name);      // "doSomething"
console.log(doAnotherThing.name);   // "doAnotherThing"
  • function names account for lots of stuff like console.log(doSomething.bind().name); // "bound doSomething" also they have "get" or "set" at the start if they are getters or setters


From: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators

  • An iterator is a thing that has a next method and keeps track of its own position
  • An iterable is something that can supply an iterator to iterate over its own values
  • The iterable has a method on the property Symbol.iterator which generates a new iterator each time it is called (for example, it could be a generator function)
  • Strings, Arrays, Maps and Sets are all iterable
  • An iterator's next() method returns a {value: "foo", done: false} or {value: undefined, done: true}, "done" will come after the last value

Generators are just like Python generators.

  • No code in the generator is run until the first time next() is called
  • You can pass values in with next(value) (just like Python coroutines)
  • The first value passed to next() is ignored because the generator is just starting out
  • Subsequent values passed to next() are substituted to the value of yield something