How can I check whether a variable is defined in Node.js?

Question

I am working on a program in node.js which is actually js.

I have a variable :

var query = azure.TableQuery...

looks this line of the

Answer 1

It sounds like you're doing property checking on an object! If you want to check a property exists (but can be values such as null or 0 in addition to truthy values), the in operator can make for some nice syntax.

var foo = { bar: 1234, baz: null };
console.log("bar in foo:", "bar" in foo); // true
console.log("baz in foo:", "baz" in foo); // true
console.log("otherProp in foo:", "otherProp" in foo) // false
console.log("__proto__ in foo:", "__proto__" in foo) // true

As you can see, the __proto__ property is going to be thrown here. This is true for all inherited properties. For further reading, I'd recommend the MDN page:

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