source: node_modules/deepmerge/index.js@ d24f17c

var defaultIsMergeableObject = require('is-mergeable-object')

function emptyTarget(val) {
        return Array.isArray(val) ? [] : {}
}

function cloneUnlessOtherwiseSpecified(value, options) {
        return (options.clone !== false && options.isMergeableObject(value))
                ? deepmerge(emptyTarget(value), value, options)
                : value
}

function defaultArrayMerge(target, source, options) {
        return target.concat(source).map(function(element) {
                return cloneUnlessOtherwiseSpecified(element, options)
        })
}

function getMergeFunction(key, options) {
        if (!options.customMerge) {
                return deepmerge
        }
        var customMerge = options.customMerge(key)
        return typeof customMerge === 'function' ? customMerge : deepmerge
}

function getEnumerableOwnPropertySymbols(target) {
        return Object.getOwnPropertySymbols
                ? Object.getOwnPropertySymbols(target).filter(function(symbol) {
                        return Object.propertyIsEnumerable.call(target, symbol)
                })
                : []
}

function getKeys(target) {
        return Object.keys(target).concat(getEnumerableOwnPropertySymbols(target))
}

function propertyIsOnObject(object, property) {
        try {
                return property in object
        } catch(_) {
                return false
        }
}

// Protects from prototype poisoning and unexpected merging up the prototype chain.
function propertyIsUnsafe(target, key) {
        return propertyIsOnObject(target, key) // Properties are safe to merge if they don't exist in the target yet,
                && !(Object.hasOwnProperty.call(target, key) // unsafe if they exist up the prototype chain,
                        && Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
}

function mergeObject(target, source, options) {
        var destination = {}
        if (options.isMergeableObject(target)) {
                getKeys(target).forEach(function(key) {
                        destination[key] = cloneUnlessOtherwiseSpecified(target[key], options)
                })
        }
        getKeys(source).forEach(function(key) {
                if (propertyIsUnsafe(target, key)) {
                        return
                }

                if (propertyIsOnObject(target, key) && options.isMergeableObject(source[key])) {
                        destination[key] = getMergeFunction(key, options)(target[key], source[key], options)
                } else {
                        destination[key] = cloneUnlessOtherwiseSpecified(source[key], options)
                }
        })
        return destination
}

function deepmerge(target, source, options) {
        options = options || {}
        options.arrayMerge = options.arrayMerge || defaultArrayMerge
        options.isMergeableObject = options.isMergeableObject || defaultIsMergeableObject
        // cloneUnlessOtherwiseSpecified is added to `options` so that custom arrayMerge()
        // implementations can use it. The caller may not replace it.
        options.cloneUnlessOtherwiseSpecified = cloneUnlessOtherwiseSpecified

        var sourceIsArray = Array.isArray(source)
        var targetIsArray = Array.isArray(target)
        var sourceAndTargetTypesMatch = sourceIsArray === targetIsArray

        if (!sourceAndTargetTypesMatch) {
                return cloneUnlessOtherwiseSpecified(source, options)
        } else if (sourceIsArray) {
                return options.arrayMerge(target, source, options)
        } else {
                return mergeObject(target, source, options)
        }
}

deepmerge.all = function deepmergeAll(array, options) {
        if (!Array.isArray(array)) {
                throw new Error('first argument should be an array')
        }

        return array.reduce(function(prev, next) {
                return deepmerge(prev, next, options)
        }, {})
}

module.exports = deepmerge