ColdFusion: how to check whether JSON property is null?

Question

Let's say I POST this simple JSON payload:

{"foo":null}

On the ColdFusion server, how do I check if the 'foo' property is null?

IsDefined won't work because it will be false for null values. IsNull won't work because IsNull will be true for not just null values, but also for missing properties.

<cfset json = DeserializeJSON(GetHttpRequestData().content) />
<cfdump var="#IsDefined("json.foo")#" /> <!--- false --->
<cfdump var="#IsNull(json.foo)#" /> <!--- true --->
<cfdump var="#IsNull(json.bar)#" /> <!--- true --->

Answer 1

My mistake, I thought null in JSON would be deserialized to empty string, but it's not true.

null in JSON is translated to be struct with key foo but undefined in CF10. (not sure about older CF version)

Therefore, a true isStructValueNull() can be written like this:

function isStructValueNull(struct, key) {
    return listFind(structKeyList(struct), key) 
             && !structKeyExists(struct, key);
}

json = deserializeJSON('{"foo":null,"bar":123}');

writeDump(isStructValueNull(json, "foo"));    // yes
writeDump(isStructValueNull(json, "bar"));    // no

or you can loop through json and use structKeyExists(), if it's false, it's null.

function structNullKeyList(struct) {
    var nulls = "";
    for (var key in struct) 
       if (!structKeyExists(struct, key))
         nulls = listAppend(nulls, key);
    return nulls;
}

writeDump(structNullKeyList(json));           // 'foo'