what does the syntax () => {} mean in Node.js

Question

I have been reading articles about Node.js and MQTT. And I am strange to the syntax as:

client.on('connect', () => {
  // do something
})

the () => {} thing, is it like function() {} ready for callback? Or does it have some specific usage? Is there some reference about that?

Answer 1

It's a shorthand for

client.on('connect', function() {
    // do something
});

Answer 2

That is the ES6 arrow function syntax. It is similar to function() {}, along with lexically binding this.

(param1, param2, …, paramN) => { statements }
(param1, param2, …, paramN) => expression
         // equivalent to:  => { return expression; }

// Parentheses are optional when there's only one parameter:
(singleParam) => { statements }
singleParam => { statements }

// A function with no parameters requires parentheses:
() => { statements }