ERC-20 Approve Facet

Approves token spending on behalf of an owner

Key Features

• Exposes approve function for managing token allowances.
• Emits Approval event for off-chain tracking.
• Interacts with diamond's shared storage for approval state.
• Functions are callable through the diamond proxy.

Overview

This facet exposes external functions for approving token spending within a diamond. It interacts with shared diamond storage to manage approvals and emits events for off-chain consumption. Developers add this facet to enable token allowance functionality in a composable manner.

Storage

ERC20Storage

Definition

struct ERC20Storage {
  mapping(address owner => uint256 balance) balanceOf;
  uint256 totalSupply;
  mapping(address owner => mapping(address spender => uint256 allowance)) allowance;
}

State Variables

Property	Type	Description
STORAGE_POSITION	bytes32	Diamond storage slot position for this module (Value: keccak256("erc20"))

Functions

approve

Approves a spender to transfer up to a certain amount of tokens on behalf of the caller. Emits an Approval event.

function approve(address _spender, uint256 _value) external returns (bool);

Parameters:

Property	Type	Description
_spender	address	The address approved to spend tokens.
_value	uint256	The number of tokens to approve.

Returns:

Property	Type	Description
-	bool	True if the approval was successful.

---

exportSelectors

Exports the function selectors of the ERC20ApproveFacet This function is use as a selector discovery mechanism for diamonds

function exportSelectors() external pure returns (bytes memory);

Returns:

Property	Type	Description
-	bytes	selectors The exported function selectors of the ERC20ApproveFacet

Events

Emitted when an approval is made for a spender by an owner.

Signature:

event Approval(address indexed _owner, address indexed _spender, uint256 _value);

Parameters:

Property	Type	Description
_owner	address	The address granting the allowance.
_spender	address	The address receiving the allowance.
_value	uint256	The amount approved.

Errors

Thrown when the spender address is invalid (e.g., zero address).

Signature:

error ERC20InvalidSpender(address _spender);

Best Practices

Best Practice

• Ensure the ERC20ApproveFacet is properly initialized within the diamond.
• Verify that the caller has the necessary permissions to approve token spending.
• Listen for Approval events to track token allowances off-chain.

Security Considerations

Security

The approve function is protected by access control, ensuring only the token owner can set allowances. Input validation for the _spender address is performed, reverting with ERC20InvalidSpender if it is the zero address. The function follows the checks-effects-interactions pattern to mitigate reentrancy risks. Developers must ensure the diamond's access control mechanisms are correctly configured for this facet.

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Last updated:March 13, 2026