Explanation, "this" syntax JavaScript

Question

So I was making a node app that had a conditional statement in the class constructor, if this then variable is equal to that, else variable equal to that. However, when I initialize that variable I want to change depending on the boolean statement with "this" syntax it gives me an error in the console. Why does this happen? And how can I initialize that variable if I can't initialize it more than once with "this" syntax in JS?

Update:

I was given How does 'this' work in JavaScript? to see if this helps, and it has nothing to do with what I asked. To be more specific, it's like why can I do this var test; if(true){var test = 1}else{var test = 0} and not this if(true){this.test = 1}else{this.test = 0} and be able to use the variable all throughout my app.

code:

class Wallet {
  constructor(secret) {
      //ADDED SECRET PARAMETER
    this.secret = secret;
    
    this.balance = STARTING_BALANCE;

    if(this.secret === null || undefined)
    {
        this.keyPair = ec.genKeyPair();
    
        this.publicKey = this.keyPair.getPublic().encode('hex');
    }
    else
    {
        this.keyPair = ec.keyFromPrivate(this.secret);
        
        this.storeKeys = this.keyPair.toString('hex');
        
        //fs.writeFileSync('../secret.json');
        this.publicKey = this.keyPair.getPublic().encode('hex');
    }

    
  }

console error:

/home/main/public_html/Cypher-Network/node_modules/bn.js/lib/bn.js:622
    var w = this.words[this.length - 1];
                      ^

TypeError: Cannot read property '-1' of null
    at BN.bitLength (/home/main/public_html/Cypher-Network/node_modules/bn.js/lib/bn.js:622:23)
    at Point._hasDoubles (/home/main/public_html/Cypher-Network/node_modules/elliptic/lib/elliptic/curve/base.js:332:48)
    at Point.mul (/home/main/public_html/Cypher-Network/node_modules/elliptic/lib/elliptic/curve/short.js:426:17)
    at KeyPair.getPublic (/home/main/public_html/Cypher-Network/node_modules/elliptic/lib/elliptic/ec/key.js:61:26)
    at new Wallet (/home/main/public_html/Cypher-Network/src/wallet/index.js:26:39)
    at Object.<anonymous> (/home/main/public_html/Cypher-Network/src/blockchain/dataBlock.js:4:16)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)

Does this answer your question? [How does 'this' work in JavaScript?](https://stackoverflow.com/questions/1197822/how-does-this-work-in-javascript) — evolutionxbox, Feb 17 '21 at 01:42
so for future references and less confusion, what I wrote works as it is intended and it's not a misusing of this in JS, right epascarello? — , Feb 17 '21 at 01:45
research JS variable SCOPE, you need to understand scoping before the `this` var in JS makes sense and why and how its used. — Dan, Feb 17 '21 at 02:01

score 0 · Accepted Answer · answered Feb 17 '21 at 01:47

0

The first problem is your conditional:

this.secret === null || undefined

should be

this.secret === null || this.secret === undefined

if you want to be explicit but you could just use a double equals instead of triple for a truthy answer that would include null or undefined:

this.secret == null

The second problem, which makes it difficult to answer your question, is it's totally unclear what this.words is, what you're trying to get the length of, and why.

If you're trying to truncate the last letter of the variable this.words, for example, you would do:

const w = this.words.splice(0, this.words.length - 1)

Only strings and arrays have a length, unless you've added a length to your own class's prototype. So this.length wouldn't work unless this is a string or an array or a class with a custom length method.

answered Feb 17 '21 at 01:47

selfagency

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Thank you for useful information, I will get back to this after experimenting on some things. – Feb 17 '21 at 01:53
it all derived from my silly conditional mistake. Thank you for answering this question, I have no eyes. – Feb 17 '21 at 01:58
2

I feel like StackOverflow would eliminate a lot of downvotes by setting up a new site that's explicitly just for getting a second set of eyes on your code to spot dumb self-inflicted bugs. – selfagency Feb 17 '21 at 02:05

score 0 · Answer 2 · answered Feb 17 '21 at 02:04

I am Assuming that your are calling your class in some other file using a import statement and passing it a random string inside a " ".

If that is the case then you need to handle that input inside your class Wallet and sanitize it for the expected input..

somehow somewhere the given input is having a break code such as \n which is the cause of this error.. maybe a use of typeof() to check that you have a valid string before this line of code

this.keyPair = ec.keyFromPrivate(this.secret);

can prevent the error..

and yes the problem in previous answer are there too..

Explanation, "this" syntax JavaScript

2 Answers2