Allow null, undefined, NaN as _value_ (NOT _keys_) for Enum type

[nodkz]

According to last changes in #812 Forbid 'true', 'false' and 'null' as key names for Enum due spec:

EnumValue : Name but not true, false or null

I got some problems with graphql-compose-elasticsearch (GraphQL wrapper for Elastic API) where some arguments are defined as GraphQLEnums and can contain booleans. So the problem was resolved by providing for boolean values specific keys boolean_true and boolean_false:

function generateEnum(typeName, vals) {
      const values = vals.reduce(
        (result, val) => {
          if (val === '') {
            result.EMPTY_STRING = { value: '' };
+          } else if (val === 'true' || val === true) {
+            result.BOOLEAN_TRUE = { value: true };
+          } else if (val === 'false' || val === false) {
+            result.BOOLEAN_FALSE = { value: false };
+          } else if (val === 'null' || val === null) {
+            result.NULL_VALUE = { value: null };
          } else if (Number.isFinite(val)) {
            result[`NUMBER_${val}`] = { value: val };
          } else {
            result[val] = { value: val };
          }
          return result;
        },
        {}
      );

      return new GraphQLEnumType({
        name: typeName,
        values,
      });
}

It works something like this:
> generateEnum('ComplexEnum', [1, 2, 3, true, false]); 
< enum ComplexEnum {
<   NUMBER_1
<   NUMBER_2
<   NUMBER_3
<   BOOLEAN_TRUE
<   BOOLEAN_FALSE
< }

But this solution does not work for null. Also, it will not work for undefined and NaN as values for enum, cause internally used isNullish method type/definition.js#L979:

function defineEnumValues(type, valueMap) {
    ...
    return {
      name: valueName,
      value: isNullish(value.value) ? valueName : value.value,
    };
}

So if I provide for GraphQLEnum such values

{
  BOOLEAN_FALSE: { value: false },  // will return `false` in resolve method
  NULL_VALUE: { value: null },      // will return `NULL_VALUE` in resolve method instead of `null`
}

I suppose that there should be another check in the helper method defineEnumValues, which should use valueName as value only if property/key value.value is not defined in object.

      value: value.hasOwnProperty('value') ? value.value : valueName

I did not found any restriction in spec that Enum values (not its keys) can not be false, null, undefined or NaN.

Or maybe I something missed?

@leebyron if i'm right, then will be nice to allow nullish values in 0.9.5 ASAP (cause v0.9.4 brings for me some breaking changes with ENUMs, and my suggested changes can also break someone's code). Better to introduce breaking changes for ENUM in one pass, cause not so many devs have yet updated to 0.9.4.

[leebyron]

Yeah - let's not use isNullish for checking enum enum value definitions.

[leebyron]

btw, names are case sensitive, so you could name them NULL or FALSE and not encounter the name collision