全部 / 前端 / 技术 · 2022年4月17日 0

7 – Array 的静态方法

广告位招租 (vx: ghostcode, 备注:网站广告)

原文地址:https://dev.to/bhagatparwinder/array-methods-in-js-33pe

Array 实用的静态方法

存在于 Array 构造函数上的方法:

Array.of

根据传入的参数创建数组

const x = Array.of("Parwinder", "Bhagat");
console.log(x); // [ 'Parwinder', 'Bhagat' ] 

// Example with the spread operator

const y = Array.of(..."Parwinder");
console.log(y); // [ 'P', 'a', 'r', 'w', 'i', 'n', 'd', 'e', 'r' ]

注意: 此方法与 Array 构造函数创建数组是有区别的,当只有一个参数时 Array 构造函数返回一个包含 n 个 undefined 的数组,而 Array.of 只返回一个包含那个参数的数组:

Array.of(7); // [7]
Array(7); // array of 7 empty slots

Array.of(1, 2, 3); // [1, 2, 3]
Array(1, 2, 3);    // [1, 2, 3]

polyfill:

if (!Array.of) {
  Array.of = function () {
    return Array.prototype.slice.call(arguments);
  };
}

Array.from

根据一个长度、类目、类数组或者可迭代的对象来创建数组。

const x = Array.from({ length: 3 });
console.log(x); // [ undefined, undefined, undefined ]

// With items using a callback function as second parameter
const y = Array.from({ length: 3 }, function () {
    return "Hello";
});
console.log(y); // [ 'Hello', 'Hello', 'Hello' ]


// With items using a callback function and arguments
const y = Array.from({ length: 3 }, function (value, index) {
    return index;
});
console.log(y); // [ 0, 1, 2 ]

polyfill:

if (!Array.from) {
  Array.from = (function () {
    var toStr = Object.prototype.toString;
    var isCallable = function (fn) {
      return typeof fn === 'function' || toStr.call(fn) === '[object Function]';
    };
    var toInteger = function (value) {
      var number = Number(value);
      if (isNaN(number)) {
        return 0;
      }
      if (number === 0 || !isFinite(number)) {
        return number;
      }
      return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
    };
    var maxSafeInteger = Math.pow(2, 53) - 1;
    var toLength = function (value) {
      var len = toInteger(value);
      return Math.min(Math.max(len, 0), maxSafeInteger);
    };

    // The length property of the from method is 1.
    return function from(arrayLike /*, mapFn, thisArg */) {
      // 1. Let C be the this value.
      var C = this;

      // 2. Let items be ToObject(arrayLike).
      var items = Object(arrayLike);

      // 3. ReturnIfAbrupt(items).
      if (arrayLike == null) {
        throw new TypeError(
          'Array.from requires an array-like object - not null or undefined'
        );
      }

      // 4. If mapfn is undefined, then let mapping be false.
      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
      var T;
      if (typeof mapFn !== 'undefined') {
        // 5. else
        // 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
        if (!isCallable(mapFn)) {
          throw new TypeError(
            'Array.from: when provided, the second argument must be a function'
          );
        }

        // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
        if (arguments.length > 2) {
          T = arguments[2];
        }
      }

      // 10. Let lenValue be Get(items, "length").
      // 11. Let len be ToLength(lenValue).
      var len = toLength(items.length);

      // 13. If IsConstructor(C) is true, then
      // 13. a. Let A be the result of calling the [[Construct]] internal method
      // of C with an argument list containing the single item len.
      // 14. a. Else, Let A be ArrayCreate(len).
      var A = isCallable(C) ? Object(new C(len)) : new Array(len);

      // 16. Let k be 0.
      var k = 0;
      // 17. Repeat, while k < len… (also steps a - h)
      var kValue;
      while (k < len) {
        kValue = items[k];
        if (mapFn) {
          A[k] =
            typeof T === 'undefined'
              ? mapFn(kValue, k)
              : mapFn.call(T, kValue, k);
        } else {
          A[k] = kValue;
        }
        k += 1;
      }
      // 18. Let putStatus be Put(A, "length", len, true).
      A.length = len;
      // 20. Return A.
      return A;
    };
  })();
}

Array.isArray()

typeof 一个数组返回的是 object ,这就是为何 isArray 出现的原因。

const x = ["Hello", "World"];
console.log(Array.isArray(x)); // true

const y = 27;
console.log(Array.isArray(y)); // false

polyfill:

if (!Array.isArray) {
    Array.isArray = function(arg) {
      return Object.prototype.toString.call(arg) === '[object Array]';
    };
}