Console Logging

  • Similar to Hardhat’s console functions.
  • You can use it in calls and transactions. It also works with view and pure functions.
  • It always works, regardless of the call or transaction failing or being successful.
  • To use it you need import it:
    • import "forge-std/console.sol";
  • You can call console.log with up to 4 parameters in any order of following types:
    • uint
    • string
    • bool
    • address
  • There’s also the single parameter API for the types above, and additionally bytes, bytes1… up to bytes32:
    • console.logInt(int i)
    • console.logUint(uint i)
    • console.logString(string memory s)
    • console.logBool(bool b)
    • console.logAddress(address a)
    • console.logBytes(bytes memory b)
    • console.logBytes1(bytes1 b)
    • console.logBytes2(bytes2 b)
    • …
    • console.logBytes32(bytes32 b)
  • console.log implements the same formatting options that can be found in Hardhat’s console.log.
    • Example: console.log("Changing owner from %s to %s", currentOwner, newOwner)
  • console.log is implemented in standard Solidity and it is compatible Anvil and Hardhat Networks.
  • console.log calls can run in other networks, like mainnet, kovan, ropsten, etc. They do nothing in those networks, but do spend a minimal amount of gas.

console.log(format[,...args])

The console.log() method prints a formatted string using the first argument as a printf-like format string which can contain zero or more format specifiers. Each specifier is replaced with the converted value from the corresponding argument. Supported specifiers are:

  • %s: String will be used to convert all values to a human-readable string. uint256, int256 and bytes values are converted to their 0x hex encoded values.
  • %d: Number will be used to convert all values to a human-readable string. This is identical to %s.
  • %i: Works the same way as %d.
  • %e: The exponential representation of a number. For uint256 and int256 types.
  • %x: The hexadecimal representation of a number. For uint256 and int256 types.
  • %o: Object. A string representation of an object with generic JavaScript-styled object formatting. For solidity types, this basically surround the string representation of the value in single-quotes.
  • %%: single percent sign (‘%’). This does not consume an argument.
  • Returns: <string> The formatted string

If a specifier does not have a corresponding argument, it is not replaced:

console.log("%s:%s", "foo");
// Returns: "foo:%s"

Values that are not part of the format string are formatted using as a human-readable string representation.

If there are more arguments passed to the console.log() method than the number of specifiers, the extra arguments are concatenated to the returned string, separated by spaces:

console.log("%s:%s", "foo", "bar", "baz");
// Returns: "foo:bar baz"

If only one argument is passed to console.log(), it is returned as it is without any formatting:

console.log("%% %s");
// Returns: "%% %s"

The String format specifier (%s) should be used in most cases unless specific functionality is needed from other format specifiers.