How to check if object property exists with a variable holding the property name?

You can use hasOwnProperty, but based on the reference you need quotes when using this method:

if (myObj.hasOwnProperty('myProp')) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

Another way is to use in operator, but you need quotes here as well:

if ('myProp' in myObj) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Thank you for everyone's assistance and pushing to get rid of the eval statement. Variables needed to be in brackets, not dot notation. This works and is clean, proper code.

Each of these are variables: appChoice, underI, underObstr.

if(typeof tData.tonicdata[appChoice][underI][underObstr] !== "undefined"){
    //enter code here
}

For own property :

var loan = { amount: 150 };
if(Object.prototype.hasOwnProperty.call(loan, "amount")) 
{ 
   //will execute
}

Note: using Object.prototype.hasOwnProperty is better than loan.hasOwnProperty(..), in case a custom hasOwnProperty is defined in the prototype chain (which is not the case here), like

var foo = {
      hasOwnProperty: function() {
        return false;
      },
      bar: 'Here be dragons'
    };

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

To include inherited properties in the finding use the in operator: (but you must place an object at the right side of 'in', primitive values will throw error, e.g. 'length' in 'home' will throw error, but 'length' in new String('home') won't)

const yoshi = { skulk: true };
const hattori = { sneak: true };
const kuma = { creep: true };
if ("skulk" in yoshi) 
    console.log("Yoshi can skulk");

if (!("sneak" in yoshi)) 
    console.log("Yoshi cannot sneak");

if (!("creep" in yoshi)) 
    console.log("Yoshi cannot creep");

Object.setPrototypeOf(yoshi, hattori);

if ("sneak" in yoshi)
    console.log("Yoshi can now sneak");
if (!("creep" in hattori))
    console.log("Hattori cannot creep");

Object.setPrototypeOf(hattori, kuma);

if ("creep" in hattori)
    console.log("Hattori can now creep");
if ("creep" in yoshi)
    console.log("Yoshi can also creep");

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Note: One may be tempted to use typeof and [ ] property accessor as the following code which doesn't work always ...

var loan = { amount: 150 };

loan.installment = undefined;

if("installment" in loan) // correct
{
    // will execute
}

if(typeof loan["installment"] !== "undefined") // incorrect
{
    // will not execute
}

A much more secure way to check if property exists on the object is to use empty object or object prototype to call hasOwnProperty()

var foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // always returns false

// Use another Object's hasOwnProperty and call it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

// It's also possible to use the hasOwnProperty property from the Object
// prototype for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

Reference from MDN Web Docs - Object.prototype.hasOwnProperty()

there are much simpler solutions and I don't see any answer to your actual question:

"it's looking for myObj.myProp but I want it to check for myObj.prop"

1. to obtain a property value from a variable, use bracket notation.
2. to test that property for truthy values, use optional chaining
3. to return a boolean, use double-not / bang-bang / (!!)
4. use the in operator if you are certain you have an object and only want to check for the existence of the property (true even if prop value is undefined). or perhaps combine with nullish coalescing operator ?? to avoid errors being thrown.

var nothing = undefined;
var obj = {prop:"hello world"}
var myProp = "prop";

consolelog( 1,()=> obj.myProp); // obj does not have a "myProp"
consolelog( 2,()=> obj[myProp]); // brackets works
consolelog( 3,()=> nothing[myProp]); // throws if not an object
consolelog( 4,()=> obj?.[myProp]); // optional chaining very nice ⭐️⭐️⭐️⭐️⭐️
consolelog( 5,()=> nothing?.[myProp]); // optional chaining avoids throwing
consolelog( 6,()=> nothing?.[nothing]); // even here it is error-safe
consolelog( 7,()=> !!obj?.[myProp]); // double-not yields true
consolelog( 8,()=> !!nothing?.[myProp]); // false because undefined
consolelog( 9,()=> myProp in obj); // in operator works
consolelog(10,()=> myProp in nothing); // throws if not an object
consolelog(11,()=> myProp in (nothing ?? {})); // safe from throwing
consolelog(12,()=> myProp in {prop:undefined}); // true (prop key exists even though its value undefined)

// beware of 'hasOwnProperty' pitfalls
// it can't detect inherited properties and 'hasOwnProperty' is itself inherited
// also comparatively verbose
consolelog(13,()=> obj.hasOwnProperty("hasOwnProperty")); // DANGER: it yields false 
consolelog(14,()=> nothing.hasOwnProperty("hasOwnProperty")); // throws when undefined
obj.hasOwnProperty = ()=>true; // someone could overwrite it
consolelog(15,()=> obj.hasOwnProperty(nothing)); // DANGER: the foolish overwrite will now always return true
consolelog(16,()=> Object.prototype.hasOwnProperty.call(obj,"hasOwnProperty")); //😭 explain?!
consolelog(17,()=> Object.hasOwn(obj,"hasOwnProperty")); //😭 explain?!

function consolelog(num,myTest){
    try{
    console.log(num,myTest());
    }
    catch(e){
    console.log(num,'throws',e.message);
    }
}

You can use hasOwnProperty() as well as in operator.

Using the new Object.hasOwn method is another alternative and it's intention is to replace the Object.hasOwnProperty method.

This static method returns true if the specified object has the indicated property as its own property or false if the property is inherited or does not exist on that object.

Please not that you must check the Browser compatibility table carefully before using this in production since it's still considered an experimental technology and is not fully supported yet by all browsers (soon to be though)

var myObj = {};
myObj.myProp = "exists";

if (Object.hasOwn(myObj, 'myProp')) {
  alert("yes, i have that property");
}

More about Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Object.hasOwn browser compatibility - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

Several ways to check if an object property exists.

const dog = { name: "Spot" }

if (dog.name) console.log("Yay 1"); // Prints.
if (dog.sex) console.log("Yay 2"); // Doesn't print. 

if ("name" in dog) console.log("Yay 3"); // Prints.
if ("sex" in dog) console.log("Yay 4"); // Doesn't print.

if (dog.hasOwnProperty("name")) console.log("Yay 5"); // Prints.
if (dog.hasOwnProperty("sex")) console.log("Yay 6"); // Doesn't print, but prints undefined.

Working for me.

if (typeof receviedData?.d?.heartbeat_interval != "undefined") {
}

In the answers I didn't see the !! truthiness check.

if (!!myObj.myProp) //Do something