When should I use `return` in es6 Arrow Functions?

问题:

The new es6 arrow functions say return is implicit under some circumstances:

The expression is also the implicit return value of that function.

In what cases do I need to use return with es6 arrow functions?

回答1:

Jackson has partially answered this in a similar question:

Implicit return, but only if there is no block.

• This will result in errors when a one-liner expands to multiple lines and the programmer forgets to add a return.
• Implicit return is syntactically ambiguous. (name) => {id: name}returns the object {id: name}... right? Wrong. It returns undefined. Those braces are an explicit block. id: is a label.

I would add to this the definition of a block:

A block statement (or compound statement in other languages) is used to group zero or more statements. The block is delimited by a pair of curly brackets.

Examples:

// returns: undefined
// explanation: an empty block with an implicit return
((name) => {})() 

// returns: \'Hi Jess\'
// explanation: no block means implicit return
((name) => \'Hi \' + name)(\'Jess\')

// returns: undefined
// explanation: explicit return required inside block, but is missing.
((name) => {\'Hi \' + name})(\'Jess\')

// returns: \'Hi Jess\'
// explanation: explicit return in block exists
((name) => {return \'Hi \' + name})(\'Jess\') 

// returns: undefined
// explanation: a block containing a single label. No explicit return.
// more: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label
((name) => {id: name})(\'Jess\') 

// returns: {id: \'Jess\'}
// explanation: implicit return of expression ( ) which evaluates to an object
((name) => ({id: name}))(\'Jess\') 

// returns: {id: \'Jess\'}
// explanation: explicit return inside block returns object
((name) => {return {id: name}})(\'Jess\') 

回答2:

I understand this rule-of-thumb ...

For functions that are effectively transforms (one-line-manipulations of arguments), return is implicit.

Candidates are:

// square-root 
value => Math.sqrt(value)

// sum
(a,b) => a+b

For other operations (more than one-liners that require a block, return has to be explicit

回答3:

There\'s another case here.

When writing a functional component in React, you can use parentheses to wrap implicitly returned JSX.

const FunctionalComponent = () => (
  <div>
    <OtherComponent />
  </div>
);

回答4:

Arrow functions allow you to have an implicit return: values are returned without having to use the return keyword.

It works when there is a on-line statement in the function body:

const myFunction = () => \'test\'

console.log(myFunction()) //\'test\'

Another example, returning an object (remember to wrap the curly brackets in parentheses to avoid it being considered the wrapping function body brackets):

const myFunction = () => ({value: \'test\'})

console.log(myFunction()) //{value: \'test\'}