有条件地检查一个有时可能不确定的值是否不等于另一个值？

Question

var query = [], name = 'name';

if (body.displayName !== req.user.displayName) {
    query.push({
        displayName: name
    });
}

This works, but I've run into a use-case where req.user.displayName === undefined .

Intuitively, it would seem undefined would also not be equal to body.displayName , but of course it throws an error:

"cannot read property of undefined"

Is there anyway to have this conditionally check if the value is undefined and also not equal to body.displayName within the same if statement, or is the only way to nest this within another if statement:

if (req.user.displayName !== undefined) { }

Answer 1

That error cannot come from req.user.displayName being undefined . It comes from trying to read a property of something that's undefined, which means that the culprit is that body , req or req.user is undefined .

Here's a transcript from node.js:

> body = {displayName: "foo"}
{ displayName: 'foo' }
> req = {user: {displayName: undefined}}
{ user: { displayName: undefined } }
> req.user
{ displayName: undefined }
> req.user.displayName
undefined
> body.displayName !== req.user.displayName
true

No problems at all, even though req.user.displayName is undefined .

You don't need to change your if test; you need to track down why one of those other objects is undefined .

Answer 2

So figure out what is undefined

console.log(req);
console.log(req.user);

I am assuming the bug is that user is not set. If that can be true, than you do not have an issue. If it should have a user than you have a bug.

But the basic check would use a truthy check to make sure the object s there. It could be either one of the following depending what is undefined.

if (req && req.user && req.user.displayName !== body.displayName)

or

if (req.user && req.user.displayName !== body.displayName)

or if body is undefined , you would need to do the check the other way.