Non-empty string being converted to "falsy" value?-CodePudding

I'm a beginner learning React/Express and ran into a problem. I figured out the cause of the problem but I'm still confused. Basically, when a user logs into the application, I create an access token using JsonWebToken and store it in the session storage. Then, I make a post request with axios, passing data and also passing the access token in the headers. I do this in the following function:

const addComment = () => {
    if (newComment){
      axios
      .post(`http://localhost:3001/comments/${id}`, {comment: newComment}, {headers: {accessToken: sessionStorage.getItem("accessToken")}})
      .then(response => {
        setComments(response.data);
      }).catch(error => {
        console.log(error);
      });
      setNewComment("");
    }
  }

The request is then handled in the backend, first through a middleware:

const { verify } = require("jsonwebtoken");

const validateToken = (req, res, next) => {
  const accessToken = req.header("accessToken");

  console.log(req.header("accessToken")); // correctly returns token
  console.log(typeof accessToken); // returns 'string'
  console.log(accessToken === true); // returns 'false'

  if (!accessToken) {
    return res.json({error: "User not logged in"});
  }

  try{
    const validToken = verify(accessToken, "importantSecret");

    if (validToken){
      return next();
    }

  } catch(err){
    return res.json({error: err});
  }
}

module.exports = { validateToken };

I used console.log() statements for the sake of debugging. When I log the value of accessToken to the console, I (as expected) get the following:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VybmFtZSI6Im9saXZlciIsImlkIjoiNjMwYWUzYjRkMmRiZmUyZGE5NDZiNmRkIiwiaWF0IjoxNjYxNzA5ODAxfQ.oVm1bIY6J5lxnaubeUh-fWWnIObmiUuL1LXjRbrW_mQ

The problem was that even though accessToken is a non-empty string, the expression accessToken === true was still evaluating to false and !accessToken was stil evaluating to true, causing the function to return an error. I now know how to fix the problem but I would really like to know why this is happening. Answers would be appreciated!

CodePudding user response：

The token is not equal to true it is equal to the token value. What you want is to check if there is a token so you would do if(token) not if(token === true).

So console.log('hi' === true) will log false because the string 'hi' does not equal true it equals 'hi'

Check how javascript equality and sameness works here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness