underscore.js 530 -- 652行

原创 于 2017-07-17 16:42:34 发布 · 268 阅读 · 0 ·
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#函数

本文介绍了一组实用的数组操作函数，包括铺平数组、去重、求并集等，通过具体的实现代码展示了如何高效地处理数组数据。

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  // Internal implementation of a recursive `flatten` function.
  var flatten = function(input, shallow, strict, output) {
    output = output || [];
    var idx = output.length;
    for (var i = 0, length = getLength(input); i < length; i++) {
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
        // Flatten current level of array or arguments object.
        if (shallow) {
          var j = 0, len = value.length;
          while (j < len) output[idx++] = value[j++];
        } else {
          flatten(value, shallow, strict, output);
          idx = output.length;
        }
      } else if (!strict) {
        output[idx++] = value;
      }
    }
    return output;
  };

  // Flatten out an array, either recursively (by default), or just one level.
  _.flatten = function(array, shallow) {
    return flatten(array, shallow, false);
  };

  // Return a version of the array that does not contain the specified value(s).
  _.without = restArgs(function(array, otherArrays) {
    return _.difference(array, otherArrays);
  });

  // Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  _.uniq = _.unique = function(array, isSorted, iteratee, context) {
    if (!_.isBoolean(isSorted)) {
      context = iteratee;
      iteratee = isSorted;
      isSorted = false;
    }
    if (iteratee != null) iteratee = cb(iteratee, context);
    var result = [];
    var seen = [];
    for (var i = 0, length = getLength(array); i < length; i++) {
      var value = array[i],
          computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted) {
        if (!i || seen !== computed) result.push(value);
        seen = computed;
      } else if (iteratee) {
        if (!_.contains(seen, computed)) {
          seen.push(computed);
          result.push(value);
        }
      } else if (!_.contains(result, value)) {
        result.push(value);
      }
    }
    return result;
  };

  // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  _.union = restArgs(function(arrays) {
    return _.uniq(flatten(arrays, true, true));
  });

  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  _.intersection = function(array) {
    var result = [];
    var argsLength = arguments.length;
    for (var i = 0, length = getLength(array); i < length; i++) {
      var item = array[i];
      if (_.contains(result, item)) continue;
      var j;
      for (j = 1; j < argsLength; j++) {
        if (!_.contains(arguments[j], item)) break;
      }
      if (j === argsLength) result.push(item);
    }
    return result;
  };

  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  _.difference = restArgs(function(array, rest) {
    rest = flatten(rest, true, true);
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    });
  });

  // Complement of _.zip. Unzip accepts an array of arrays and groups
  // each array's elements on shared indices.
  _.unzip = function(array) {
    var length = array && _.max(array, getLength).length || 0;
    var result = Array(length);

    for (var index = 0; index < length; index++) {
      result[index] = _.pluck(array, index);
    }
    return result;
  };

  // Zip together multiple lists into a single array -- elements that share
  // an index go together.
  _.zip = restArgs(_.unzip);

  // Converts lists into objects. Pass either a single array of `[key, value]`
  // pairs, or two parallel arrays of the same length -- one of keys, and one of
  // the corresponding values. Passing by pairs is the reverse of _.pairs.
  _.object = function(list, values) {
    var result = {};
    for (var i = 0, length = getLength(list); i < length; i++) {
      if (values) {
        result[list[i]] = values[i];
      } else {
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

flatten

 // Internal implementation of a recursive `flatten` function.
  var flatten = function(input, shallow, strict, output) {
    output = output || [];
    var idx = output.length;
    for (var i = 0, length = getLength(input); i < length; i++) {
      var value = input[i];
      if (isArrayLike(value) && (_.isArray(value) || _.isArguments(value))) {
        // Flatten current level of array or arguments object.
        if (shallow) {
          var j = 0, len = value.length;
          while (j < len) output[idx++] = value[j++];
        } else {
          flatten(value, shallow, strict, output);
          idx = output.length;
        }
      } else if (!strict) {
        output[idx++] = value;
      }
    }
    return output;
  };

  // Flatten out an array, either recursively (by default), or just one level.
  _.flatten = function(array, shallow) {
    return flatten(array, shallow, false);
  };

flatten函数铺平
网上搜着帮忙了
这里写图片描述

搜索网址

_.without

_.without = restArgs(function(array, otherArrays) {
    return _.difference(array, otherArrays);
  });
_.difference = restArgs(function(array, rest) {
    rest = flatten(rest, true, true);
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    });
  });

这个_.without

arguments[rest] ⇒  [[1,2], [1,3]]

会对otherarrays铺平一层数组吗然后一个个探测是否目标数组不具有对比数组元素，具有返回false，有返回true，最终返回符合条件的元素。
而_.difference则只是使用方式不同罢了

arguments[1] ⇒ [[1,2], [1,3]]

_.unique

// Produce a duplicate-free version of the array. If the array has already
  // been sorted, you have the option of using a faster algorithm.
  // Aliased as `unique`.
  _.uniq = _.unique = function(array, isSorted, iteratee, context) {
    if (!_.isBoolean(isSorted)) {
      context = iteratee;
      iteratee = isSorted;
      isSorted = false;
    }
    if (iteratee != null) iteratee = cb(iteratee, context);
    var result = [];
    var seen = [];
    for (var i = 0, length = getLength(array); i < length; i++) {
      var value = array[i],
          computed = iteratee ? iteratee(value, i, array) : value;
      if (isSorted) {
        if (!i || seen !== computed) result.push(value); // 如果sorted了，则seen为单独的一个值
        seen = computed;
      } else if (iteratee) { //如果没有，则seen作为一个数组，每次必会被对照，而且value组也会跟sorted一样被对照，运算效率上会低一些
        if (!_.contains(seen, computed)) {
          seen.push(computed);
          result.push(value);
        }
      } else if (!_.contains(result, value)) {
        result.push(value);
      }
    }
    return result; //返回结果
  };

_.union

 // Produce an array that contains the union: each distinct element from all of
  // the passed-in arrays.
  _.union = restArgs(function(arrays) {
    return _.uniq(flatten(arrays, true, true));
  });

参数名都叫arrays，应该夸名字起得好，作者的意思都不需要看代码了。

_.intersection


  // Produce an array that contains every item shared between all the
  // passed-in arrays.
  _.intersection = function(array) {
    var result = []; //输出结果
    var argsLength = arguments.length; 
    for (var i = 0, length = getLength(array); i < length; i++) {
      var item = array[i]; // array是一个数组
      if (_.contains(result, item)) continue; //结果已有则跳过
      var j;
      for (j = 1; j < argsLength; j++) {
        if (!_.contains(arguments[j], item)) break; //不存在则无需检测，并非交集
      }
      if (j === argsLength) result.push(item); // 检测完毕，说明是交集，将之加入结果。
    }
    return result;
  };

_.difference


  // Take the difference between one array and a number of other arrays.
  // Only the elements present in just the first array will remain.
  _.difference = restArgs(function(array, rest) {
    rest = flatten(rest, true, true); // 先铺平rest
    return _.filter(array, function(value){
      return !_.contains(rest, value);
    }); ／／ 再筛选出值
  });

_.unzip


  // Complement of _.zip. Unzip accepts an array of arrays and groups
  // each array's elements on shared indices.
  _.unzip = function(array) {
    var length = array && _.max(array, getLength).length || 0;
    var result = Array(length); //找到最长的数组

    for (var index = 0; index < length; index++) {
      result[index] = _.pluck(array, index); // 取每个数组的0,1,2,3.......属性并进行归类
    }
    return result;
  };

_.zip

  _.zip = restArgs(_.unzip);

我们再回顾一下restArg函数

var restArgs = function(func, startIndex) {
    startIndex = startIndex == null ? func.length - 1 : +startIndex;
    return function() {
      var length = Math.max(arguments.length - startIndex, 0),
          rest = Array(length),
          index = 0;
      for (; index < length; index++) {
        rest[index] = arguments[index + startIndex];
      }
      switch (startIndex) {
        case 0: return func.call(this, rest);
        case 1: return func.call(this, arguments[0], rest);
        case 2: return func.call(this, arguments[0], arguments[1], rest);
      }
      var args = Array(startIndex + 1);
      for (index = 0; index < startIndex; index++) {
        args[index] = arguments[index];
      }
      args[startIndex] = rest;
      return func.apply(this, args);
    };
  };

我们发现，当我们传入unzip的结果，也就是在arguments中传数组的时候，unzip就会将array默认为rest param，进行处理，也就是说，比如我们传入了以0分组的数组，当我们匹配了第一个数组的array［0］，即原本第一个数组的第一个数array[0]时则其会相应匹配array[1]分组中的第一个，即原本第一个数组的array[1]，这样就又达到了合并数据的效果, 非常巧妙。

_.object

 // Converts lists into objects. Pass either a single array of `[key, value]`
  // pairs, or two parallel arrays of the same length -- one of keys, and one of
  // the corresponding values. Passing by pairs is the reverse of _.pairs.
  _.object = function(list, values) {
    var result = {};
    for (var i = 0, length = getLength(list); i < length; i++) {
      if (values) {
        result[list[i]] = values[i];
      } else {
        result[list[i][0]] = list[i][1];
      }
    }
    return result;
  };

以上一个例子就可以明白这个想做什么了

_.object(['moe', 'larry', 'curly'], [30, 40, 50]);
=> {moe: 30, larry: 40, curly: 50}

_.object([['moe', 30], ['larry', 40], ['curly', 50]]);
=> {moe: 30, larry: 40, curly: 50}

没了
这里写图片描述

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