`throw` as an expression?

[Axel Rauschmayer]

Use case: With promises, the expression body form of arrow functions is so convenient. Alas, `throw` being a statement, you can?t use it there. For example, the following code is not syntactically legal:

```js
asyncFunc()
.then(count => count >= 0 ? count : throw new Error(...))
.then(...)
.catch(...);
```

Could `throw` be turned into an expression?
--
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de



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[Andrea Giammarchi]

probably not exactly the fat arrow usage you were looking for ... but it
makes it trivial to inline any sort of block

```js
asyncFunc()
.then(count => count >= 0 ? count : ()=>{throw new Error(...)}())
.then(...)
.catch(...);
```

Regards

Post by Axel Rauschmayer
Use case: With promises, the expression body form of arrow functions is so
convenient. Alas, `throw` being a statement, you can?t use it there. For
```js
asyncFunc()
.then(count => count >= 0 ? count : throw new Error(...))
.then(...)
.catch(...);
```
Could `throw` be turned into an expression?
--
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de
_______________________________________________
es-discuss mailing list
es-discuss at mozilla.org
https://mail.mozilla.org/listinfo/es-discuss

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[John Lenz]

or a simple utility method


On Thu, Oct 9, 2014 at 6:35 AM, Andrea Giammarchi <

Post by Andrea Giammarchi
probably not exactly the fat arrow usage you were looking for ... but it
makes it trivial to inline any sort of block
```js
asyncFunc()
.then(count => count >= 0 ? count : ()=>{throw new Error(...)}())
.then(...)
.catch(...);
```
Regards

Post by Axel Rauschmayer
Use case: With promises, the expression body form of arrow functions is
so convenient. Alas, `throw` being a statement, you can?t use it there. For
```js
asyncFunc()
.then(count => count >= 0 ? count : throw new Error(...))
.then(...)
.catch(...);
```
Could `throw` be turned into an expression?
--
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de
_______________________________________________
es-discuss mailing list
es-discuss at mozilla.org
https://mail.mozilla.org/listinfo/es-discuss

_______________________________________________
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https://mail.mozilla.org/listinfo/es-discuss

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[Nathan Wall]

Post by John Lenz
or a simple utility method

Promise.reject could be used.

```js
asyncFunc()
.then(count => count >= 0 ? count : Promise.reject(new Error(...)))
.then(...)
.catch(...)
```

Nathan

[Brendan Eich]

Post by John Lenz
or a simple utility method

Current keyword-anchored statement form does not require parens around
expression to evaluate and throw.

/be

[John Lenz]

Sure, but there is no pressing need here, "return" as an expression would
be more interesting. Not that I'm asking for that.

Post by Brendan Eich

Post by John Lenz
or a simple utility method

Current keyword-anchored statement form does not require parens around
expression to evaluate and throw.
/be

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[Isiah Meadows]

It would be convenient for throw to be one. Maybe an idea would be to
treat it similar to `void` or `delete`? If it were an expression, it
could be parsed very similarly. Here's a potential use case even
without all the other ES6 additions.

```js
function foo(bar) {
bar == null && throw new TypeError();
// do stuff with required parameter `bar`
}

// Currently, it requires this:
function foo(bar) {
if (bar == null) {
throw new TypeError();
}
// do stuff with required parameter `bar`
}

// Or, this: (totally stealing your hack, Andrea... :-P)
const foo = bar => bar == null ?
()=>{throw new TypeError()} :
undefined; /* do something */
```

It wouldn't surprise me if this becomes included eventually in, say,
Underscore/Lodash/etc. if not made an expression. It would just end up
easier to type `_.throw(new TypeError())`.

To clarify, here's what I mean by treating it like `void` and `delete`:

```js
void foo(); //=> returns undefined
delete bar[0]; //=> returns if the entry/property no longer exists
throw baz; //=> returns nothing/undefined (for sake of spec)?
```

From: Andrea Giammarchi <andrea.giammarchi at gmail.com>
To: Axel Rauschmayer <axel at rauschma.de>
Cc: es-discuss list <es-discuss at mozilla.org>
Date: Thu, 9 Oct 2014 14:35:50 +0100
Subject: Re: `throw` as an expression?
probably not exactly the fat arrow usage you were looking for ... but it makes it trivial to inline any sort of block
```js
asyncFunc()
.then(count => count >= 0 ? count : ()=>{throw new Error(...)}())
.then(...)
.catch(...);
```
Regards

Post by Axel Rauschmayer
```js
asyncFunc()
.then(count => count >= 0 ? count : throw new Error(...))
.then(...)
.catch(...);
```
Could `throw` be turned into an expression?
--
Dr. Axel Rauschmayer
axel at rauschma.de
rauschma.de
_______________________________________________
es-discuss mailing list
es-discuss at mozilla.org
https://mail.mozilla.org/listinfo/es-discuss

_______________________________________________
es-discuss mailing list
es-discuss at mozilla.org
https://mail.mozilla.org/listinfo/es-discuss