Proposal: a more consistent and stricter number converting function - Number.of()

[Claude Pache]

> Le 24 févr. 2017 à 04:50, 段垚 <duanyao at ustc.edu> a écrit :
> 
> Hi,
> 
> 
> Converting an arbitray value to a number in JS can be rather inconsistent and unexpected:
> 
> * `null` and `undefined` are different: `+null === 0` but `+undefined` is NaN.
> 
> * Empty string and non-nubmeric strings are different: `+"" === 0` but `+"foo"` is NaN.
> 
> 
> This problem can be worse because JSON only support finite numbers:
> 
> ```
> 
> var total = 0;
> 
> total += JSON.parse(JSON.stringify({ "value": 0/0 })).value;
> 
> total === 0; //Oops, NaN is serialized as null, and then converted to 0
> 
> ```
> 
> So I propose a more consistent and stricter number converting function: `Number.of(value)`:
> 
> 1. If `value` is `null` or `undefined`, return `NaN`;
> 
> 2. If `value` is a number, return `value` itself;
> 
> 3. If `value.valueOf()` returns a number, return that number, otherwise return `NaN`.
> 
> 
> This means all non-number values except those have number type `.valueOf()` would be converted to NaN:
> 
> 
> ```
> 
> Number.of(null); // NaN
> 
> Number.of(''); //NaN
> 
> Number.of('1'); //NaN
> 
> 
> var total = 0;
> 
> total += Number.of(JSON.parse(JSON.stringify({ "value": 0/0 })).value);
> 
> total; // NaN
> 
> ```
> 
> 
> What do you think?
> 
> 
> Regards,
> 
> Duan, Yao
> 


Depending on the concrete situation, you might not need yet another way to convert into number.

* If you know that your input is either a string or null/undefined (e.g., as the result of  `someHTMLElement.getAttribute('foo')`), you could use `Number.parseFloat()`, which will produce NaN for the empty string, null and undefined.

* If your issue is precisely with null/undefined, as it is the case in your JSON example, a more generic solution would be the null-coalescing operator `??`, which allows to express more precisely and more clearly what you mean. The semantics is:

```js
a ?? b // evaluates `a`. If `a` is null or undefined, evaluates `b`.
```

In your JSON example:

```js
var total = 0;

total += JSON.parse(JSON.stringify({ "value": 0/0 })).value ?? NaN;

Number.isNaN(total); // true. Hurray!
```

—Claude