Arrow function without curly braces

Actually in a briefcase when somebody uses braces in an arrow function declaration, it is equal to below:

const arrow = number => number + 1;

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const arrow = (number) => number + 1;

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const arrow = (number) => ( number + 1 );

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const arrow = (number) => { return number + 1 };

The parenthesis are returning a single value, the curly braces are executing multiple lines of code.

Your example looks confusing because it's using JSX which looks like multiple "lines" but really just gets compiled to a single "element."

Here are some more examples that all do the same thing:

const a = (who) => "hello " + who + "!";
const b = (who) => ("hello " + who + "!");
const c = (who) => (
  "hello " + who + "!"
);
const d = (who) => (
  "hello "
    + who
    + "!"
);
const e = (who) => {
  return "hello " + who + "!";
}; 

You will also often see parenthesis around object literals because that's a way to avoid the parser treating it as a code block:

const x = () => {} // Does nothing
const y = () => ({}) // returns an object

Parenthesis are used in an arrow function to return an object.

() => ({ name: 'YourName' })  // This will return an object

That is equivalent to

() => {
   return { name : 'YourName' }
}

One can also use curly braces to prevent a single line arrow function from returning a value -- or to make it obvious to the next developer that a single line arrow function shouldn't, in this case, be returning anything.

For example:

const myFunc = (stuff) => { someArray.push(stuff) }
const otherFunc = (stuff) => someArray.push(stuff)

console.log(myFunc())    // --> logs undefined
console.log(otherFunc()) // --> logs result of push which is new array length