JavaScript中常见的Polyfill示例详解

if (!Array.prototype.includes) {
  Array.prototype.includes = function(searchElement, fromIndex) {
    if (this == null) {
      throw new TypeError('"this" is null or undefined');
    }
    
    var o = Object(this);
    var len = o.length >>> 0;
    
    if (len === 0) {
      return false;
    }
    
    var n = fromIndex | 0;
    var k = Math.max(n >= 0 ? n : len + n, 0);
    
    while (k < len) {
      if (o[k] === searchElement) {
        return true;
      }
      k++;
    }
    return false;
  };
}

if (!window.Promise) {
  window.Promise = function(executor) {
    // 简化版 Promise 实现
    var self = this;
    self.status = 'pending';
    self.data = undefined;
    self.onResolvedCallback = [];
    self.onRejectedCallback = [];
    
    function resolve(value) {
      if (self.status === 'pending') {
        self.status = 'resolved';
        self.data = value;
        for(var i = 0; i < self.onResolvedCallback.length; i++) {
          self.onResolvedCallback[i](value);
        }
      }
    }
    
    // ... 其他实现细节
  };
}

// 只引入需要的 polyfill
import 'core-js/features/array/includes';
import 'core-js/features/promise';

// .babelrc
{
  "presets": [
    ["@babel/preset-env", {
      "useBuiltIns": "usage",
      "corejs": 3
    }]
  ]
}