What is msg.value?

Question

Can somebody explain what is the magic msg.value? I came across this code: https://learnxinyminutes.com/docs/solidity/

/// @return The balance of the user after the deposit is made
function deposit() public payable returns (uint) {
    // Use 'require' to test user inputs, 'assert' for internal invariants
    // Here we are making sure that there isn't an overflow issue
    require((balances[msg.sender] + msg.value) >= balances[msg.sender]);

    balances[msg.sender] += msg.value;
    // no "this." or "self." required with state variable
    // all values set to data type's initial value by default

    LogDepositMade(msg.sender, msg.value); // fire event

    return balances[msg.sender];
}

I do not see where the msg.value coming from, when a user deposit eth, does he or she needs to pass in the amount from the deposit function? Thanks and sorry for newbie question.

Edit:

From the following contract, I do not see anywhere passing in msg.value, so how this msg.value has been assigned? by the way you link of cryptozombie is very interesting, thanks.

pragma solidity ^0.4.0;

contract ClientReceipt {

    event Deposit(
        address indexed _from,
        bytes32 indexed _id,
        uint _value
    );

    function deposit(bytes32 _id) public payable {
        // Events are emitted using `emit`, followed by
        // the name of the event and the arguments
        // (if any) in parentheses. Any such invocation
        // (even deeply nested) can be detected from
        // the JavaScript API by filtering for `Deposit`.
        emit Deposit(msg.sender, _id, msg.value);
    }
}

http://solidity.readthedocs.io/en/develop/contracts.html#function-modifiers

Answer 1

msg.value is a member of the msg (message) object when sending (state transitioning) transactions on the Ethereum network.

msg.value contains the amount of wei (ether / 1e18) sent in the transaction.

msg.data (bytes): complete calldata
msg.gas (uint): remaining gas - deprecated in version 0.4.21 and to be replaced by gasleft()
msg.sender (address): sender of the message (current call)
msg.sig (bytes4): first four bytes of the calldata (i.e. function identifier)
msg.value (uint): number of wei sent with the message

Answer 2

msg.value is automatically set to the amount of ether sent with that payable function.

More info here: http://solidity.readthedocs.io/en/develop/units-and-global-variables.html?highlight=msg.value#block-and-transaction-properties

Answer 3

You can define value parameter while seding the transaction .Below example can give better explanation

   ContractInterface.methods
      .contractmethod(parameter)
       .send({
         from: accounts[0],
         value : "amount in ether or wei"
    });

Above value will be available as msg.value

Answer 4

It is a parameter users send along when they call a function in smart contract. It is the amount of wei you send to the smart contract. If you use remix you will see there is a input labeled "value" to put this parameter.

If you call the function from javascript you can put this parameter as Arjun recommended above. ContractInterface.methods .contractmethod(parameter) .send({ from: accounts[0], value : "amount in ether or wei" });

You said that you are a newbie, so the article might be quite difficult to understand. I recommend that you should read easier articles than that one. You can start with https://solidity.readthedocs.io/en/v0.4.25/solidity-in-depth.html or other tutorials.