Can javascript variables contain anything?

Any value can be stored in a JavaScript variable. Values include:

• All primitives
• Object references

Almost everything is an object in JavaScript. Notably, functions are objects (including React stateless functional components [SFCs] and React components, which are constructor functions), and so a variable can contain a reference to a function:

function foo(msg) {
    console.log("foo: " + msg);
}
function bar(msg) {
    console.log("bar: " + msg);
}
const f = Math.random() < 0.5 ? foo : bar;
f("Hi");

You have a 50/50 chance of seeing "foo: Hi" or "bar: Hi" with that code.

There are only a few things I can think of that a variable cannot contain:

• Operators. E.g. this is not valid:

  // NOT VALID
  const op = Math.random() < 0.5 ? + : -;
  console.log(10 op 5);
  

  ...although it's easy to get the same effect with functions:

  const op = Math.random() < 0.5
             ? (a, b) => a + b
             : (a, b) => a - b;
  console.log(op(10, 5));
  

• Execution contexts or lexical environments, but only because nothing ever exposes a reference to them in code.

• Memory locations, because (again) nothing ever exposes them in code.

...and even json

JSON is a textual notation for data exchange. (More here.) If you're dealing with JavaScript source code, and not dealing with a string, you're not dealing with JSON.

Variables are the way to address all objects. JavaScript is an Object-based language (String, Number, etc. are all Objects with different prototypes).

For example, a var can hold: true, undefined, null, "string", `template`, {}, [], NaN, 7, infinity, /abc/

There aren'y any limitations I can think of.