数组映射返回未定义的数组时应返回对象数组

Question

Why does

['a', 'b', 'c'].map((x) => { letter: x }) returns a array of undefined

and

['a', 'b', 'c'].map((x) => [{ letter: x }][0]) returns a array of objects correctly?

Answer 1

You need to wrap object in ()

 var result = ['a', 'b', 'c'].map((x) => ({ letter: x })) console.log(result) 

Answer 2

Because

• You use the curly brackets as block statement .

• You have letter as a label .

• x is just a value without some action.

• The return of undefined is the standard return value of a function without any return statement with value.

  To return a value other than the default, a function must have a return statement that specifies the value to return. A function without a return statement will return a default value. In the case of a constructor called with the new keyword, the default value is the value of its this parameter. For all other functions, the default return value is undefined.

Correct call for mapping objects.

 console.log(['a', 'b', 'c'].map(x => ({ letter: x }))); 

Answer 3

A function that lacks an explicit return statement will return undefined. () => {} is equivalent to function(){} . x => x is equivalent to function(x){ return x;}

So arrow function without {} will return computed value of the expression.