Question

{
    "@": {
        "xmlns": "http://ses.amazonaws.com/doc/2010-12-01/"
    },
    "SendEmailResult": {
        "MessageId": "0000012fdd10caaf-021c6e9e-e872-4b35-ad94-1d11c79a6324-000000"
    },
    "ResponseMetadata": {
        "RequestId": "736d5bb2-7b7d-11e0-b435-f7b0c9315f0d"
    }
}

How do I check if “MessageId” exists in a object? (without throwing errors)
I might get other json objects returned, and I need to know if the one I get has a “MessageId”.

Answer 1

Assuming you have a reference to it in obj, then:

if (obj && obj.SendEmailResult && "MessageId" in obj.SendEmailResult) {
    // The "MessageId" property exists in `obj.SendEmailResult`
}

Probably more usefully, though:

var msgid = obj && obj.SendEmailResult && obj.SendEmailResult.MessageId;
if (msgid) {
    // The property exists and is "truthy", `msgid` is the value
}

JavaScript’s AND operator && is more useful than that of some other languages, it returns the right-hand side’s value if both of its operands are “truthy” (rather than just returning a true / false result, as in most languages). A “truthy” value is a value that is not “falsy” (obviously). “Falsy” values are false, undefined, null, "", and 0.

So the above basically says “Set msgid to obj.SendEmailResult.MessageId provided that obj and obj.SomeEmailResult both exist.

(The || operator is similarly powerful.)