index.js

var invariant = require('invariant');

var hasOwnProperty = Object.prototype.hasOwnProperty;

var assign = Object.assign || /* istanbul ignore next */ function assign(target, source) {
  getAllKeys(source).forEach(function(key) {
    if (hasOwnProperty.call(source, key)) {
      target[key] = source[key];
    }
  });
  return target;
};

var getAllKeys = typeof Object.getOwnPropertySymbols === 'function' ?
  function(obj) { return Object.keys(obj).concat(Object.getOwnPropertySymbols(obj)) } :
  /* istanbul ignore next */ function(obj) { return Object.keys(obj) };

/* istanbul ignore next */
function copy(object) {
  if (object instanceof Array) {
    return assign(object.constructor(object.length), object)
  } else if (object && typeof object === 'object') {
    var prototype = object.constructor && object.constructor.prototype
    return assign(Object.create(prototype || null), object);
  } else {
    return object;
  }
}

function newContext() {
  var commands = assign({}, defaultCommands);
  update.extend = function(directive, fn) {
    commands[directive] = fn;
  };
  update.isEquals = function(a, b) { return a === b; };

  return update;

  function update(object, spec) {
    if (!(Array.isArray(object) && Array.isArray(spec))) {
      invariant(
        !Array.isArray(spec),
        'update(): You provided an invalid spec to update(). The spec may ' +
        'not contain an array except as the value of $set, $push, $unshift, ' +
        '$splice or any custom command allowing an array value.'
      );
    }

    invariant(
      typeof spec === 'object' && spec !== null,
      'update(): You provided an invalid spec to update(). The spec and ' +
      'every included key path must be plain objects containing one of the ' +
      'following commands: %s.',
      Object.keys(commands).join(', ')
    );

    var nextObject = object;
    var index, key;
    getAllKeys(spec).forEach(function(key) {
      if (hasOwnProperty.call(commands, key)) {
        var objectWasNextObject = object === nextObject;
        nextObject = commands[key](spec[key], nextObject, spec, object);
        if (objectWasNextObject && update.isEquals(nextObject, object)) {
          nextObject = object;
        }
      } else {
        var nextValueForKey = update(object[key], spec[key]);
        if (!update.isEquals(nextValueForKey, nextObject[key])) {
          if (nextObject === object) {
            nextObject = copy(object);
          }
          nextObject[key] = nextValueForKey;
        }
      }
    })
    return nextObject;
  }

}

var defaultCommands = {
  $push: function(value, nextObject, spec) {
    invariantPushAndUnshift(nextObject, spec, '$push');
    if (value.length) {
      nextObject = copy(nextObject)
      nextObject.push.apply(nextObject, value)
    }
    return nextObject;
  },
  $unshift: function(value, nextObject, spec) {
    invariantPushAndUnshift(nextObject, spec, '$unshift');
    if (value.length) {
      nextObject = copy(nextObject)
      nextObject.unshift.apply(nextObject, value)
    }
    return nextObject;
  },
  $splice: function(value, nextObject, spec, originalObject) {
    invariantSplices(nextObject, spec);
    value.forEach(function(args) {
      invariantSplice(args);
      if (nextObject === originalObject && args.length) nextObject = copy(originalObject);
      nextObject.splice.apply(nextObject, args);
    });
    return nextObject;
  },
  $set: function(value, nextObject, spec) {
    invariantSet(spec);
    return value;
  },
  $unset: function(value, nextObject, spec, originalObject) {
    invariant(
      Array.isArray(value),
      'update(): expected spec of $unset to be an array; got %s. ' +
      'Did you forget to wrap the key(s) in an array?',
      value
    );
    value.forEach(function(key) {
      if (Object.hasOwnProperty.call(nextObject, key)) {
        if (nextObject === originalObject) nextObject = copy(originalObject);
        delete nextObject[key];
      }
    });
    return nextObject;
  },
  $merge: function(value, nextObject, spec, originalObject) {
    invariantMerge(nextObject, value);
    getAllKeys(value).forEach(function(key) {
      if (value[key] !== nextObject[key]) {
        if (nextObject === originalObject) nextObject = copy(originalObject);
        nextObject[key] = value[key];
      }
    });
    return nextObject;
  },
  $apply: function(value, original) {
    invariantApply(value);
    return value(original);
  }
};

module.exports = newContext();
module.exports.newContext = newContext;

// invariants

function invariantPushAndUnshift(value, spec, command) {
  invariant(
    Array.isArray(value),
    'update(): expected target of %s to be an array; got %s.',
    command,
    value
  );
  var specValue = spec[command];
  invariant(
    Array.isArray(specValue),
    'update(): expected spec of %s to be an array; got %s. ' +
    'Did you forget to wrap your parameter in an array?',
    command,
    specValue
  );
}

function invariantSplices(value, spec) {
  invariant(
    Array.isArray(value),
    'Expected $splice target to be an array; got %s',
    value
  );
  invariantSplice(spec['$splice']);
}

function invariantSplice(value) {
  invariant(
    Array.isArray(value),
    'update(): expected spec of $splice to be an array of arrays; got %s. ' +
    'Did you forget to wrap your parameters in an array?',
    value
  );
}

function invariantApply(fn) {
  invariant(
    typeof fn === 'function',
    'update(): expected spec of $apply to be a function; got %s.',
    fn
  );
}

function invariantSet(spec) {
  invariant(
    Object.keys(spec).length === 1,
    'Cannot have more than one key in an object with $set'
  );
}

function invariantMerge(target, specValue) {
  invariant(
    specValue && typeof specValue === 'object',
    'update(): $merge expects a spec of type \'object\'; got %s',
    specValue
  );
  invariant(
    target && typeof target === 'object',
    'update(): $merge expects a target of type \'object\'; got %s',
    target
  );
}