code: function mergeConfig(loaderConfig) {
    var defaultConfig = {
      // Web3
      provider: 'http://localhost:8545',
  
      // For deployment
      from: web3.eth.accounts[0],
      gasLimit: web3.eth.getBlock(web3.eth.defaultBlock).gasLimit,
  
      // Specify contract constructor parameters, if any.
      // constructorParams: {
      //   ContractOne: [ 'param1_value', 'param2_value' ]
      // }
      constructorParams: {},
  
      // To use deployed contracts instead of redeploying, include contract addresses in config
      // deployedContracts: {
      //   ContractOne: '0x...........',
      //   ContractTwo: '0x...........',
      // }
      deployedContracts: {}
    };
  
    var mergedConfig = loaderConfig;
    for (var key in defaultConfig) {
      if (!mergedConfig.hasOwnProperty(key)) {
        mergedConfig[key] = defaultConfig[key];
      }
    }
    return mergedConfig;
  }
docs: Merge loaderConfig and default configurations