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手写Array.prototype.forEach

May 23, 2020

前端知识复习 code snippet

array.prototype.foreach 标准

  1. Let O be ? ToObject(this value).
  2. Let len be ? LengthOfArrayLike(O).
  3. If IsCallable(callbackfn) is false, throw a TypeError exception.
  4. Let k be 0.

  5. Repeat, while k < len,

    • Let Pk be ! ToString(k).
    • Let kPresent be ? HasProperty(O, Pk).

    • If kPresent is true, then

      • Let kValue be ? Get(O, Pk).
      • Perform ? Call(callbackfn, thisArg, « kValue, k, O »).
  6. Set k to k + 1.
  7. Return undefined.

代码

Array.prototype.myForEach = function (callbackfn, thisArg) {
    // this must not be null or undefined
    if (this == null) {
        throw new TypeError(
            'Array.prototype.forEach called on null or undefined'
        );
    }
    // callbackfn must be a function
    if (typeof callbackfn !== 'function') {
        throw new TypeError(callbackfn + ' is not a function');
    }

    var O = Object(this);
    var len = O.length >>> 0;

    var k = 0;
    // iterate array
    if (thisArg === undefined) {
        for (; k < len; k++) {
            if (k in O) {
                var kValue = O[k];
                callbackfn(kValue, k, O);
            }
        }
    } else {
        for (; k < len; k++) {
            if (k in O) {
                var kValue = O[k];
                callbackfn.call(thisArg, kValue, k, O);
            }
        }
    }

    return undefined;
};