用js实现js内置函数

实现 Array.prototype.map

Array.prototype.myMap = function(callback, thisArg) {
  if (this == null) {
    throw new TypeError('this is null or not defined');
  }
  if (typeof callback !== 'function') {
    throw new TypeError(callback + ' is not a function');
  }

  const result = [];
  const array = Object(this);
  const length = array.length >>> 0;

  for (let i = 0; i < length; i++) {
    if (i in array) {
      result[i] = callback.call(thisArg, array[i], i, array);
    }
  }
  return result;
};

实现 Array.prototype.filter

Array.prototype.myFilter = function(callback, thisArg) {
  if (this == null) {
    throw new TypeError('this is null or not defined');
  }
  if (typeof callback !== 'function') {
    throw new TypeError(callback + ' is not a function');
  }

  const result = [];
  const array = Object(this);
  const length = array.length >>> 0;

  for (let i = 0; i < length; i++) {
    if (i in array && callback.call(thisArg, array[i], i, array)) {
      result.push(array[i]);
    }
  }
  return result;
};

实现 Array.prototype.reduce

Array.prototype.myReduce = function(callback, initialValue) {
  if (this == null) {
    throw new TypeError('this is null or not defined');
  }
  if (typeof callback !== 'function') {
    throw new TypeError(callback + ' is not a function');
  }

  const array = Object(this);
  const length = array.length >>> 0;
  let accumulator = initialValue;
  let i = 0;

  if (initialValue === undefined) {
    if (length === 0) {
      throw new TypeError('Reduce of empty array with no initial value');
    }
    accumulator = array[0];
    i = 1;
  }

  for (; i < length; i++) {
    if (i in array) {
      accumulator = callback(accumulator, array[i], i, array);
    }
  }
  return accumulator;
};

实现 Function.prototype.bind

Function.prototype.myBind = function(thisArg, ...boundArgs) {
  if (typeof this !== 'function') {
    throw new TypeError('Function.prototype.bind - what is trying to be bound is not callable');
  }

  const originalFunc = this;
  return function(...args) {
    return originalFunc.apply(thisArg, boundArgs.concat(args));
  };
};

实现 Object.create

Object.myCreate = function(proto, propertiesObject) {
  if (typeof proto !== 'object' && proto !== null) {
    throw new TypeError('Object prototype may only be an Object or null');
  }

  function F() {}
  F.prototype = proto;
  const obj = new F();

  if (propertiesObject !== undefined) {
    Object.defineProperties(obj, propertiesObject);
  }

  if (proto === null) {
    obj.__proto__ = null;
  }

  return obj;
};

实现 Promise

class MyPromise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];

    const resolve = (value) => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onFulfilledCallbacks.forEach(fn => fn());
      }
    };

    const reject = (reason) => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(fn => fn());
      }
    };

    try {
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err; };

    const promise2 = new MyPromise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      } else if (this.state === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      } else {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0);
        });
        this.onRejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          }, 0);
        });
      }
    });

    return promise2;
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  if (x === promise2) {
    return reject(new TypeError('Chaining cycle detected for promise'));
  }

  if (x instanceof MyPromise) {
    x.then(resolve, reject);
  } else {
    resolve(x);
  }
}