使用箭头函数需要注意的

A minor improvement. The effect on code using Promises can be more dramatic, as the }).then(function (result) { lines can pile up.

Note that an arrow function with a block body does not automatically return a value. Use a return statement for that.

There is one caveat when using arrow functions to create plain objects. Always wrap the object in parentheses:

// create a new empty object for each puppy to play with
var chewToys = puppies.map(puppy => {}); // BUG!
var chewToys = puppies.map(puppy => ({})); // ok

Unfortunately, an empty object {} and an empty block {} look exactly the same. The rule in ES6 is that { immediately following an arrow is always treated as the start of a block, never the start of an object. The code puppy => {} is therefore silently interpreted as an arrow function that does nothing and returns undefined.

Even more confusing, an object literal like {key: value} looks exactly like a block containing a labeled statement—at least, that’s how it looks to your JavaScript engine. Fortunately { is the only ambiguous character, so wrapping object literals in parentheses is the only trick you need to remember.