纯原生仿ES6的Object.assign，实现深度合并对象

function isObj(x){
var type = typeof x;
return x !== null && (type === 'object' || type === 'function');
}

var hasOwnProperty = Object.prototype.hasOwnProperty;
var propIsEnumerable = Object.prototype.propertyIsEnumerable;

function toObject(val) {
if (val === null || val === undefined) {
throw new TypeError('Cannot convert undefined or null to object');
}

return Object(val);
}

function assignKey(to, from, key) {
var val = from[key];

if (val === undefined || val === null) {
return;
}

if (hasOwnProperty.call(to, key)) {
if (to[key] === undefined || to[key] === null) {
throw new TypeError('Cannot convert undefined or null to object (' + key + ')');
}
}

if (!hasOwnProperty.call(to, key) || !isObj(val)) {
to[key] = val;
} else {
to[key] = assign(Object(to[key]), from[key]);
}
}

function assign(to, from) {
if (to === from) {
return to;
}

from = Object(from);

for (var key in from) {
if (hasOwnProperty.call(from, key)) {
assignKey(to, from, key);
}
}

if (Object.getOwnPropertySymbols) {
var symbols = Object.getOwnPropertySymbols(from);

for (var i = 0; i < symbols.length; i++) {
if (propIsEnumerable.call(from, symbols[i])) {
assignKey(to, from, symbols[i]);
}
}
}

return to;
}

function deepAssign(target) {
target = toObject(target);

for (var s = 1; s < arguments.length; s++) {
assign(target, arguments[s]);
}

return target;
};

deepAssign({a: {b: 0}}, {a: {b: 1, c: 2}}, {a: {c: 3}});
//=> {a: {b: 1, c: 3}}