Checking for an undefined nested variable in a JS object

Question

Is there a better or shorthand way to perform a check on whether or not a variable within (possibly several levels of) an object is null or undefined?

What I would currently do to check is:

if (typeof json !== 'undefined' || typeof json.data !== 'undefined' || typeof json.data.formkeys !== 'undefined') {}

Is there a shorthand way to check for whether or not "json.data.formkeys" is undefined without the concern that one or all of the higher level properties or the object itself might not currently exist?

Answer 1

There's a lot of ways to do this. Here's an exotic (and costly) one:

if(((json || {}).data || {}).formkeys !== void 0){}

As commented, a try-catch will work

// JS has function scope. foo starts its life as declared but undefined
// If the try-catch fails, foo will remain undefined.
try{ var foo = json.data.formkeys; } catch(){}

if(foo !== undefined){}

A longer and possibly better way to do it is to create a resolver function. Most frameworks I have encountered use some form of this in their getter functions so that getting a value from a half-complete path will just return undefined instead of throwing midway.

function resolve(path, root){
  root = root || window;
  return path.split('.').reduce(function(parent, child){
    // A quick check for brevity. You can replace this with a better check
    // and return logic.
    return parent && parent[child];
  }, root);
}

var foo = resolve('json.data.formkeys');

if(foo !== undefined){}

Answer 2

This is one case where eval is not evil, so long as you guard your code:

function getValue(obj, propertyString) {
    if (!/[_$a-z0-9.]/.test(propertyString)) {
        throw new Error("Invalid propertyString");
    }

    try {
        return eval("obj." + propertyString);
    }
    catch (error) {
        return undefined;
    }
}

(sigh) A quote from Dr. Evil seems oddly appropriate for eval (Dr. Eval? ... Nah.):

Well it's true! It's true! You're semi-evil. You're quasi-evil. You're the margarine of evil. You're the Diet Coke of evil. Just one calorie, not evil enough.

Related: eval() isn’t evil, just misunderstood

Answer 3

Another one:

if ('formkeys' in ((json || {}).data || {})) {}

which is shorter than:

if (((json || {}).data || {}).formkeys !== void 0) {}