Export

This is a default import:
// B.js
import A from './A'
It only works if A contains a default export:
// A.js
export default 42

In this case it doesn't matter what name you assign to it when importing:
// B.js
import A from './A'
import MyA from './A'
import Something from './A'
Because it will always resolve to whatever is the default export of A.

This is a named import called A:
import { A } from './A'
It only works if A contains a named export called A:
export const A = 42
In this case the name matters because you're importing a specific thing by its export name:
// B.js
import { A } from './A'
import { myA } from './A' // Doesn't work!
import { Something } from './A' // Doesn't work!
To make these work, you would add a corresponding named export to A:
// A.js
export const A = 42
export const myA = 43
export const Something = 44

A module can only have one default export, but as many named exports as you like (zero, one, or many). You can import them together:
// B.js
import A, { myA, Something } from './A'
Here, we import the default export as A, and named exports called myA and Something, respectively.
// A.js
export default 42
export const myA = 43
export const Something = 44
We can also assign them all different names when importing:
// B.js
import X, { myA as myX, Something as XSomething } from './A'

參考連結

http://stackoverflow.com/questions/36795819/when-should-i-use-curly-braces-for-es6-import/36796281