UntypedError when running methods in a Javascript FT smart contract

I am a new to developing on NEAR and trying to create a FT smart contract, I haven’t learned Rust yet so I am using Javascript-Typescript but I am having this bug when I try to run some fucntion, the contract is deploying well, I can run init function well, but when I try to run other function that’s when I am getting the error below.

Here is the code:

import {
  NearBindgen,
  near,
  call,
  view,
  initialize,
  assert,
  AccountId,
} from "near-sdk-js";


interface TokenMetadata {
  name: string;
  symbol: string;
  decimals: number;
}

@NearBindgen({})
class FungibleTokenContract {
  owner_id: string;
  total_supply: number;
  metadata:TokenMetadata;
  balances: Map<string, number>;

  constructor(owner_id: string, total_supply: number, metadata: TokenMetadata) {
    this.owner_id = owner_id;
    this.total_supply = total_supply;
    this.metadata = metadata;
    this.balances = new Map<string, number>();
  }

  @initialize({ privateFunction: true })
  init(owner_id: string, total_supply: number, metadata: TokenMetadata): void {
    this.owner_id = owner_id;
    this.total_supply = total_supply;
    this.metadata = metadata;
  }

  // Method to issue new tokens to a recipient
  @call({})
  issueTokens(recipient: string, amount: number): void {
    let caller_id = near.predecessorAccountId();
    assert(
      caller_id === this.owner_id,
      "Only the contract owner can issue tokens."
    );

    let recipientBalance = this.balances.get(recipient);
    this.balances.set(recipient, recipientBalance + amount);
    this.total_supply -= amount;

    near.log(
      `Issued ${amount} tokens to account: ${recipient}. Total supply: ${this.total_supply}`
    );
  }

  // Method to retrieve token metadata
  @view({})
  tokenMetadata(): any {
    return this.metadata;
  }

  // Method to transfer tokens from sender to recipient
  @call({})
  transferTokens(sender: string, recipient: string, amount: number): void {
    let senderBalance = this.balances.get(sender);
    assert(senderBalance >= amount, "Insufficient balance.");

    this.balances.set(sender, senderBalance - amount);

    let recipientBalance = this.balances.get(recipient);
    this.balances.set(recipient, recipientBalance + amount);

    near.log(`Transferred ${amount} tokens from ${sender} to ${recipient}.`);
  }

  // Method to distribute tokens as incentives/rewards
  @call({})
  distributeIncentives(account: string, amount: number): void {
    let caller_id = near.predecessorAccountId();
    assert(
      caller_id === this.owner_id,
      "Only the contract owner can distribute incentives."
    );

    let accountBalance = this.balances.get(account);
    this.balances.set(account, accountBalance + amount);
    this.total_supply -= amount;

    near.log(
      `Distributed ${amount} tokens to account: ${account}. Total supply: ${this.total_supply}`
    );
  }

  // Method to get token balance for an account
  @view({})
  getTokenBalance(account: string): number {
    return this.balances.get(account);
  }
}

And this this is the full error message, when I run the getTokenBalance for exapmle, or the tokenMetadata :

near view $ID getTokenBalance '{"account": "'$ID'"}'
View call: dev-1685171425396-11051470732136.getTokenBalance({"account": "dev-1685171425396-11051470732136"})
An error occured
Error: Querying [object Object] failed: wasm execution failed with error: HostError(GuestPanic { panic_msg: "cannot read property 'constructor' of undefined\n    at _reconstruct (build/hello_near.js:601)\n    at getTokenBalance (build/hello_near.js:674)\n" }).
{
  "block_hash": "Cz9sV5Vf9oy8d9gv2epNyKhZzvpd4PmXWmp4kYFahDqi",
  "block_height": 127569024,
  "error": "wasm execution failed with error: HostError(GuestPanic { panic_msg: \"cannot read property 'constructor' of undefined\\n    at _reconstruct (build/hello_near.js:601)\\n    at getTokenBalance (build/hello_near.js:674)\\n\" })",
  "logs": []
}
    at JsonRpcProvider.query (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/node_modules/near-api-js/lib/providers/json-rpc-provider.js:123:19)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Account.viewFunction (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/node_modules/near-api-js/lib/account.js:366:24)
    at async exports.callViewFunction (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/index.js:103:48)
    at async Object.handler (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/utils/exit-on-error.js:52:9)
TypedError: Querying [object Object] failed: wasm execution failed with error: HostError(GuestPanic { panic_msg: "cannot read property 'constructor' of undefined\n    at _reconstruct (build/hello_near.js:601)\n    at getTokenBalance (build/hello_near.js:674)\n" }).
{
  "block_hash": "Cz9sV5Vf9oy8d9gv2epNyKhZzvpd4PmXWmp4kYFahDqi",
  "block_height": 127569024,
  "error": "wasm execution failed with error: HostError(GuestPanic { panic_msg: \"cannot read property 'constructor' of undefined\\n    at _reconstruct (build/hello_near.js:601)\\n    at getTokenBalance (build/hello_near.js:674)\\n\" })",
  "logs": []
}
    at JsonRpcProvider.query (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/node_modules/near-api-js/lib/providers/json-rpc-provider.js:123:19)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Account.viewFunction (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/node_modules/near-api-js/lib/account.js:366:24)
    at async exports.callViewFunction (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/index.js:103:48)
    at async Object.handler (/home/enoch/.nvm/versions/node/v18.12.1/lib/node_modules/near-cli/utils/exit-on-error.js:52:9) {
  type: 'UntypedError',
  context: undefined
}

Hey @iamenochchirima Whilst you are more than welcome to troubleshoot your JS errors on the governance forum you might get better community support on our Discord server. We have a number of technical channels and everyone seems very helpful

You’re definitely not alone—many blockchain developers starting out on NEAR face similar issues when using JavaScript/TypeScript SDKs. From your error and code, it looks like the problem comes down to how you’re using the Map object for balances.

In JavaScript, a Map.get() will return undefined if the key doesn’t exist, and doing arithmetic with undefined causes the kind of error you’re seeing (cannot read property 'constructor' of undefined).

Quick Fix:

Update all your Map.get() lines to provide a default value when the account doesn’t exist yet. For example, if you try to get the balance for a recipient or sender and the key doesn’t exist, you should set it to 0 instead of undefined. This ensures that when you try to do arithmetic (like adding or subtracting tokens), you don’t run into errors because undefined isn’t a valid number.

Also, make sure your init function is actually initializing balances to a new Map object if you’re not using the constructor (because in your code you have a constructor but NEAR calls init instead). So, initialize the balances map inside the init function as well.

Updated Example:

For each place in your code where you are fetching the balance of a user, be sure to default the value to 0 if the account doesn’t exist yet. For example, before doing any arithmetic with balances, check if the balance exists; if not, initialize it to 0.

Today have a Near Aí tool for dev. Maybe people of Orizon can save them and @frol @Near-Dev can support your issues.

Hi @iamenochchirima, please, join one of the devs support channels: https://dev.near.org/communities

There are several problems in your code:

1. SDK collections require exactly one parameter - the unique storage prefix. See the docs: Collections | NEAR Documentation
2. The functions must accept exactly one argument which you can deconstruct inline if you want: myFunc({ param1, param2 }: myFuncParams), but you cannot use myFunc(param1: TParam, param2: TParam) - think about it as React props. See the docs above for examples