Curly Brackets in Arrow Functions

问题

can someone, please explain the following:

I\'m following Dan Abramov\'s lectures & doing the exercises.

The code works fine, however, the tests fail when the following particular function is written with curly brackets **{ }**.

    case \'toggleTodo\' :
        return (
            state.map( (one) => {
                oneTodo( one, action )
            })
        );

The same code works fine without curly brackets.

    case \'toggleTodo\' :
        return (
            state.map( (one) => 
                oneTodo( one, action )
            )
        );

Here is the JsBin. Please refer to line 31 onwards.

回答1:

case 'toggleTodo' :
    return (
        state.map( (one) => 
            oneTodo( one, action )
        )
    );

is equal to:

case 'toggleTodo' :
    return (
        state.map( (one) => {
            return oneTodo( one, action )
        })
    );

see the return statement

回答2:

The pair of braces forms a block, containing a list of statements. You need to use a return statement explicitly to make the function return something.

If you omit the braces, the arrow function has a concise body, which consists solely of a single expression whose result will implicitly become the return value of the function.

来源：https://stackoverflow.com/questions/35440265/curly-brackets-in-arrow-functions