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')