为什么 map 方法不返回 object 的数组？

Question

let x = {a:1,b:2,c:3}
let result = Object.keys(x).map((i) => {
  console.log(i) ;
  return ({i :x[i]});
})

why result is

[{i: 1},{i: 2},{i: 3}]

? In console, value of i ie a,b,c is being printed. What happens during return?

Answer 1

Why map method does not return array of object?

It does.

What happens during return?

The return ({i:x[i]}); line means:

• Create an object.
• Give it a property called "i" (not the value of i , the actual literal name "i" ) with the value from x[i] .
• Return that as the value for this map iteration, which will be used in the result array.

The result is an array of objects, each with one property called "i" .

If you meant to use the value of i , you'd need to use a computed property name. There's also no reason for the () around the object literal:

return {[i]: x[i]};
//      ^^^------------- computed property name

Live Example:

 let x = {a:1,b:2,c:3}; let result = Object.keys(x).map((i) => { console.log(i); return {[i]: x[i]}; }); console.log(result);

 .as-console-wrapper { max-height: 100%;important; }

This was introduced in ES2015. In ES5 and earlier, you'd have to create the object first, then add the property to it afterward.