带有ES6赋值的箭头函数语法

Question

Quick question, I'm new with es6 syntax and typscript and I just found this code in my current ng2 project:

this.contextActions.getCurrentContextAction().subscribe((link) => this.currentAction = link);

Just to be clear if I understand it correctly.

The function subscribe takes as argument a function which takes a parameter (link) and it assigns the input parameter to the current action, and it returns the result of this assigment?

Is it correct my understanding? What is actually what this arrow function returns? The result of the assigment? An assigment does not return anything right?

Please clrify this to me.

Thanks

Answer 1

Is it correct my understanding? What is actually what this arrow function returns? The result of the assigment?

Yes. It may not be significant depending on what the code calling it does with the return value.

An assigment does not return anything right?

It evaluates as the assigned value.

 var bar, foo; foo = (bar = 1); console.log("Foo is " + foo); 

Answer 2

An assignment does not return anything right?

Wrong, the value of an assignment is the value that is assigned:

 var a; console.log(a = 5); 

This is why you can do things like this:

 var a, b, c; console.log(a = b = c = 5); // all have the value 5 

So the arrow function returns its parameter, link . I don't know if the subscribe function does something with the returned value. Presumably not.

The rest of your understanding is correct.

Answer 3

This is effectively the same as:

var _this = this;
this.contextActions
  .getCurrentContextAction()
  .subscribe(function(link) { 
    return _this.currentAction = link
  });

Subscribe accepts a function that takes a single parameter (link) that will be internally called by subscribe at a certain stage.

In this context, link is an internally generated variable that is given to the function you are passing in as input. You could alternatively do the following:

function myFunction(link) { 
  return _this.currentAction = link
}

var _this = this;
this.contextActions
  .getCurrentContextAction()
  .subscribe(myFunction);

The arrow function will assign the value given to your callback to this.currentAction and then return the value in a single statement.

Eg:

var a, b, c;

console.log(a = 5); // Logs out 5

function test() {
  return b = 6;
}

c = test();
console.log(b); // Logs out 6
console.log(c); // Logs out 6

Answer 4

A good way to test this is to open up a browser and see what it does. Let's see what happens in Chrome Dev tools when console log an assignment.

var someNum = 5;

console.log(someNum = 20);

20

Answer 5

If you did not use => it would look like this

var _that = this;
.subscribe(function(link){
   return _that.currentAction = link;
}

It returns the assigned value.

See the docs .

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

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