# mockFunction

Description: mockFunction cheatcode documentation

Note: This document was authored using MDX

  Source: https://github.com/NomicFoundation/hardhat-website/tree/main/src/content/docs/docs/reference/cheatcodes/Environment/mock-function.mdx

  Components used in this page:
    - :::note: An informational callout block. Supports custom title `:::note[Title]` and icon `:::note{icon="name"}` syntax.

{/* This document contains content copied/adapted from the Foundry Book (MIT licensed). See LICENSE in the parent directory. */}

### Signature

```solidity
function mockFunction(
  address callee,
  address target,
  bytes calldata data
) external;
```

### Description

Executes calls to an address `callee` with bytecode of address `target` if the call data either strictly or loosely matches `data`.

When a call is made to `callee` the call data is first checked to see if it matches in its entirety with `data`.
If not, the call data is checked to see if there is a partial match on function selector.

If a match is found, then call is executed using the bytecode of `target` address.

:::note
**Isolated tests**

This cheatcode does not currently work if using isolated test mode.
:::

### Examples

For two contracts (with same storage layout):

```solidity
contract Counter {
  uint256 public a;

  function count(uint256 x) public {
    a = 321 + x;
  }
}

contract ModelCounter {
  uint256 public a;

  function count(uint256 x) public {
    a = 123 + x;
  }
}
```

Mocking an exact call to `count` function:

```solidity
function testMockFunction() public {
  vm.mockFunction(
    address(counter),
    address(model),
    abi.encodeWithSelector(Counter.count.selector, 456)
  );
  counter.count(456);
  assertEq(counter.a(), 123 + 456);
  counter.count(567);
  assertEq(counter.a(), 321 + 567);
}
```

Mocking all calls to `count` function:

```solidity
function testMockCall() public {
  vm.mockFunction(
    address(counter),
    address(model),
    abi.encodeWithSelector(Counter.count.selector)
  );
  counter.count(678);
  assertEq(counter.a(), 123 + 678);
  counter.count(789);
  assertEq(counter.a(), 123 + 789);
}
```