闭包中的箭头函数

Question

Why this is working:

var a = () => { 

    var print = function(i) { console.log(i); return this; }    
    var print2 = function(i) { console.log(i); return this; }

    return { print:print , print2:print2 }
}

a().print(5).print2(5);

this is also working:

var b = () => { 

    var print = (i) => { console.log(i); return this; }    
    return { print:print}
}
b().print('Arrow function works');

while this is not working:

var b = () => { 

    var print = (i) => { console.log(i); return this; }    
    var print2 = function(i) { console.log(i); return this; }

    return { print:print , print2:print2 }
}
b().print(5).print2(5);

https://jsfiddle.net/Imabot/1gt2kxfh/14/

Answer 1

It's all due to arrow functions behavior( docs )

Step by step explanation:

 var b = () => { // 1 var print = (i) => { console.log(i); return this; } var print2 = function(i) { console.log(i); return this; } return { print:print , print2:print2 } } const res = b() // 2 const secondRes = res.print(5) // 3 secondRes.print2(5);

1. here print function saves this reference from the outer scope, so this can't be reassigned anymore
2. now print function is not using this reference that comes from res variable, because this has already been attached to print function above
3. as a result secondRes is not going to reference to the object that was returned by b function. But it will use this reference that is attached to print function. And finally because secondRes doesn't have print2 property - it throws

Hope it helps <3

Answer 2

In a non-arrow function, the value of this depends on how the function is called. If the function is called as a member of an object, this refers to this object:

someObj.myFunction() // inside myFunction this will point to someObj

In contrast, the arrow functions do not affect this . So whithin an arrow function the value of this is whatever it is in the enclosing scope.

Answer 3

The answer from Lex82 gives the why. If you want to return the functions, so you can use function chaining:

 var b = () => { var print = (i) => { console.log(i); return { print:print , print2:print2 }; } var print2 = function(i) { console.log(i); return this; } return { print:print , print2:print2 } } b().print(5).print2(5);