Are these lines of JavaScript code equivalent?

I've found this string in JavaScript code.

var c = (a.b !== null) ? a.b : null;

This is a shorthand of an if-else statement, however the value null is assigned if it is null. Isn't that ALWAYS equivalent to

var c = a.b

including all cases - exceptions, null, undefined, etc?

In another words, are these lines (always) equivalent?

var c = (a.b !== null) ? a.b : null;

-vs-

var c = a.b

No, they AREN'T NECESSARILY EQUAL always if b is a getter that updates a variable. It's bad practice to code this way though

var log = 0;
var a = {
    get b() {
        log++;
        return log;
    }
}

var c = (a.b !== null) ? a.b : null;
// outputs 2
console.log(c);

var log = 0;
var a = {
    get b() {
        log++;
        return log;
    }
}

var c = a.b;
// outputs 1
console.log(c);

These statements are logically equivalent.

That being said, and as mentioned in another answer, if a.b has side effects, the statements will not result in the same program state.

This could be readily obvious in the form of var c having a different value depending on which of these statements are executed, or more hidden, if a.b modifies something elsewhere in the program.

Refactoring

As refactoring has been discussed, I'll touch on it briefly. As the above has hopefully made obvious, directly refactoring would not be safe in all scenarios. However, I would still recommend a refactor of one kind or another.

The two possible situations as I see them are:

1. a.b has no side effects, direct refactoring is safe
2. a.b has hidden side effects. This represents very unclear, confusing, and just downright bad code. It should be refactored so that all changes happening during the statement are clear and obvious to a reader (hopefully intuitively so, as well as supported by comments).

As @potatopeelings already pointed out, the two possible statements are not always equivalent, since one can write obscure code, which will have different results.

However, if I see a code, like

var c = (a.b !== null) ? a.b : null;

I will assume that the code's intention is

var c = a.b;

so I will change it to make the code prettier. If I will be negatively surprised, that is, the code does not pass the testing phases due to this change, then I will try to find the author of a.b with git blame.

So, my answer is, that the two statements are not equivalent, but should be equivalent in well-written code.

Well, actually not even

var c = (a !== null) ? a : null;

is guaranteed to be equivalent to

var c = a;

when a is resolved by a getter or an ES6 proxy handler on the global object.

Hence for instance, this assign to c the value 0:

Object.defineProperty(self, 'a', { get: function() { return c ^= 1; } });
var c = (a !== null) ? a : null;
console.log(c);

while this assigns to c the value 1:

Object.defineProperty(self, 'a', { get: function() { return c ^= 1; } });
var c = a;
console.log(c);

You're right, var c = a.b is exactly the same as var c = (a.b !== null) ? a.b : null;

My guess is the null in the ternary operator was meant to be anything except null, a default value if you will.

The reason for that confusingly odd syntax is because a.b might be an empty string OR undefined, and apparently an empty string is valid input.

Also, note: a.b might be a function.