NEP-17

NEP-17 is a Neo Improvement Proposal that defines a token standard for Neo smart contracts.

Properties

According to the NEP-17 standard a NEP-17 smart contract must have the following properties:

• decimals
• symbol
• name

The standard actually calls them "methods", but for NEO•ONE smart contracts they will be public readonly properties which compile down to methods.

Let’s take a look at an example:

export class Token extends SmartContract {
  public readonly name = 'Eon';
  public readonly symbol = 'EON';
  public readonly decimals = 8;
}

Methods

According to the NEP-17 standard a NEP-17 smart contract must have the following methods:

• balanceOf
• totalSupply
• transfer

Let’s take a look at an example:

export class Token extends SmartContract {
  @constant
  public get totalSupply(): Fixed<8> {
    return this.mutableSupply;
  }

  @constant
  public balanceOf(address: Address): Fixed<8> {
    const balance = this.balances.get(address);

    return balance === undefined ? 0 : balance;
  }

  public transfer(from: Address, to: Address, amount: Fixed<8>): boolean {
    // Sanity check that amount.
    if (amount < 0) {
      throw new Error(`Amount must be greater than 0: ${amount}`);
    }

    // Verify that the `from` `Address` has approved this call.
    if (!Address.isCaller(from)) {
      throw new Error('from Address did not approve the transfer');
    }

    // Sanity check that the `from` `Address` has enough balance
    const fromBalance = this.balanceOf(from);
    if (fromBalance < amount) {
      throw new Error('From balance is insufficient');
    }

    // Update balances accordingly
    const toBalance = this.balanceOf(to);
    this.balances.set(from, fromBalance - amount);
    this.balances.set(to, toBalance + amount);

    return true;
  }
}

Events

According to the NEP-17 standard a NEP-17 smart contract must emit the following events:

• Transfer

Let’s take a look at an example:

export class Token extends SmartContract {}

If all these criteria are met then the NEO•ONE compiler will produce a manifest with "NEP-17" included in the supportedStandards array.