在javascript映射es6中传递其他参数

Question

I'm trying to pass in an additional parameter to the callback function in a map. The value is not passed when I use es6 syntax for callback function and in map.

Here is the es6 map and callback function

 const convertEvents = action.payload.map(item => convertEvent(item), { role: 'teacher' }); const convertEvent = (item) => { console.log('----------convertEvent role----------'); console.log(this.role); return item; }; 

But when I used old javascript syntax the value is passed and the code works correctly

 const convertEvents = action.payload.map(convertEventRole, { role: 'teacher' }); function convertEventRole(item) { console.log('----------convertEvent role----------'); console.log(this.role); return item; } 

Can you tell me why the es6 code didn't work?

Answer 1

The 2nd parameter passed to Array.map() is the thisArg , which is:

Value to use as this when executing callback.

With a standard JS function , this is defined by execution context, but you can change it using Function.bind() , and other methods.

An arrow function this is defined by the context in which it's declared, and thus cannot be changed. That's why you can use the assigned thisArg with an arrow function.

You can approximate the functionality using partial application and IIFE :

 const arr = [1, 2, 3]; const result = arr.map(((m) => (n) => n + m)(5)); console.log(result);