solcjs provides a compiler method, which is used to compile solidity code. This method can be used in two different ways depending on whether the source code has any imports or not. If the source code doesn't have any imports, then it takes two arguments; that is, the first argument is solidity source code as a string and a Boolean indicating whether to optimize the byte code or not. If the source string contains multiple contracts, then it will compile all of them.

Here is an example to demonstrate this:

var solc = require("solc"); 
var input = "contract x { function g() {} }"; 
var output = solc.compile(input, 1); // 1 activates the optimiser  
for (var contractName in output.contracts) { 
    // logging code and ABI  
    console.log(contractName + ": " + output.contracts[contractName].bytecode); 
    console.log(contractName + "; " + JSON.parse(output.contracts[contractName].interface)); 
} 

If your source code contains imports, then the first argument will be an object whose keys are filenames and values are the contents of the files. So whenever the compiler sees an import statement, it doesn't look for the file in the filesystem; instead, it looks for the file contents in the object by matching the filename with the keys. Here is an example to demonstrate this:

var solc = require("solc"); 
var input = { 
    "lib.sol": "library L { function f() returns (uint) { return 7; } }", 
    "cont.sol": "import 'lib.sol'; contract x { function g() { L.f(); } }" 
}; 
var output = solc.compile({sources: input}, 1); 
for (var contractName in output.contracts) 
    console.log(contractName + ": " + output.contracts[contractName].bytecode); 

If you want to read the imported file contents from the filesystem during compilation or resolve the file contents during compilation, then the compiler method supports a third argument, which is a method that takes the filename and should return the file content. Here is an example to demonstrate this:

var solc = require("solc"); 
var input = { 
    "cont.sol": "import 'lib.sol'; contract x { function g() { L.f(); } }" 
}; 
function findImports(path) { 
    if (path === "lib.sol") 
        return { contents: "library L { function f() returns (uint) { return 7; } }" } 
    else 
        return { error: "File not found" } 
} 
var output = solc.compile({sources: input}, 1, findImports); 
for (var contractName in output.contracts) 
    console.log(contractName + ": " + output.contracts[contractName].bytecode);