why throw should be always between curly brace?

Question

I had a situation like code below and since it's an arrow function and for a single line of code in arrow functions , no curly braces are required, I didn't use that and It works just fine.

const test = new Promise((resolve, failure) => {
failure("mistake");
});
const handler = test.catch(() => console.log("it's ok"));

but in code below when I typed throw without any curly braces, it just threw an exception:

const test = new Promise((resolve, failure) => {
failure("mistake");
});
const handler = test.catch(() => throw new Error("it's not ok"));

but just after adding curly braces to throw, it started to function correctly again:

const test = new Promise((resolve, failure) => {
failure("mistake");
});
const handler = test.catch(() => {throw new Error("it's not ok")});

so I wonder why throw needs curly braces under any circumstances?

Answer 1

since it's an arrow function and for a single line of code in arrow functions , no curly braces are required

More accurately, for a single expression, no curly braces are required. The critical question is: what is an "expression"?

An expression is a piece of code that resolves to some value. So for example, 2 + 3 is an expression; it calculates and then resolves to 5. console.log("it's ok") is an expression; it calls a function then resolves to the function's return value (in this case, undefined).

But something like let x = 2 + 3 is not an expression. It does contain an expression (the 2 + 3 part), but the full thing is a "statement". Statements do not resolve to a value, and so they can't really be combined with other things For example, console.log(let x = 2 + 3) or let y = (let x = 2 + 3) are not legal.

The code throw new Error("it's not ok") is a statement; it does not resolve to a value. Thus, it cannot be used in spots that require an expression. You can't do let x = throw new Error("it's not ok"), and you can't omit the curly braces of your arrow function.