Object.assign

先看一下表现特征.

  1. 一个对象里面有嵌套对象的话, 改变其中的值会影响到双方, 改变最外层则不会
  2. Properties on the prototype chain and non-enumerable properties cannot be copied
const obj1 = { a: 0, b: { c: 0 } };
const obj2 = Object.assign({}, obj1);
console.log(obj2); // { a: 0, b: { c: 0 } }
 
obj1.a = 1;
console.log(obj1); // { a: 1, b: { c: 0 } }
console.log(obj2); // { a: 0, b: { c: 0 } }
 
obj2.b.c = 3;
console.log(obj1); // { a: 1, b: { c: 3 } }
console.log(obj2); // { a: 2, b: { c: 3 } }
 
const v1 = "abc";
const v2 = true;
const v3 = 10;
const v4 = Symbol("foo");
 
const obj3 = Object.assign({}, v1, null, v2, undefined, v3, v4);
// Primitives will be wrapped, null and undefined will be ignored.
// Note, only string wrappers can have own enumerable properties.
console.log(obj3); // { "0": "a", "1": "b", "2": "c" }

正式重写

if(!Object.assign) {
  Object.assign = function(target) {
    var newVal = Object(target);
    for(var i = 1; i < arguments.length; i ++) {
      var nextArg = arguments[i];
      // Primitives will be wrapped, null and undefined will be ignored.
      if(nextArg !== null && nextArg !== undefined) {
        for(var nextKey in nextArg) {
          if(Object.prototype.hasOwnProperty.call(nextArg, nextKey)) {
            newVal[nextKey] = nextArg[nextKey];
          }
        }
      }
    }
    return newVal;
  }
}

Object.is

考虑到两种情况:

  1. 0 === -0 // true
  2. NaN === NaN // false
if (!Object.is) {
  Object.defineProperty(Object, "is", {
    value: function (x, y) {
      // SameValue algorithm
      if (x === y) {
        // return true if x and y are not 0, OR
        // if x and y are both 0 of the same sign.
        // This checks for cases 1 and 2 above.
        return x !== 0 || 1 / x === 1 / y;
      } else {
        // return true if both x AND y evaluate to NaN.
        // The only possibility for a variable to not be strictly equal to itself
        // is when that variable evaluates to NaN (example: Number.NaN, 0/0, NaN).
        // This checks for case 3.
        return x !== x && y !== y;
      }
    }
  });
}