underscore.js源码解析集合部分
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// Collection Functions // -------------------- // The cornerstone, an `each` implementation, aka `forEach`. // Handles raw objects in addition to array-likes. Treats all // sparse array-likes as if they were dense. /* params: 数组、对象或类数组对象,函数,函数执行环境 */ _.each = _.forEach = function(obj, iteratee, context) { iteratee = optimizeCb(iteratee, context); var i, length; if (isArrayLike(obj)) {// 数组或类数组 for (i = 0, length = obj.length; i < length; i++) { iteratee(obj[i], i, obj);// item index obj } } else {// 对象 var keys = _.keys(obj);// 返回键的数组 for (i = 0, length = keys.length; i < length; i++) { iteratee(obj[keys[i]], keys[i], obj); } } return obj; }; // Return the results of applying the iteratee to each element. /* 通过转换函数(iteratee迭代器)映射列表中的每个值产生价值的新数组 这个函数很简洁的处理类数组与对象的不同情况,值得学习!!!!!!!!!!!!!!!!!!! */ _.map = _.collect = function(obj, iteratee, context) { iteratee = cb(iteratee, context); var keys = !isArrayLike(obj) && _.keys(obj),//如果是数组,返回false;否则返回对象的keys数组 length = (keys || obj).length, results = Array(length); for (var index = 0; index < length; index++) { var currentKey = keys ? keys[index] : index;// 如果是对象,返回key; 数组返回index results[index] = iteratee(obj[currentKey], currentKey, obj); } return results; }; // Create a reducing function iterating left or right. /* 创建迭代 params:方向(-1为左,1为右) */ var createReduce = function(dir) { // Wrap code that reassigns argument variables in a separate function than // the one that accesses `arguments.length` to avoid a perf hit. (#1991) /* params: obj,执行函数,起始值,起始值和context */ var reducer = function(obj, iteratee, memo, initial) { var keys = !isArrayLike(obj) && _.keys(obj), length = (keys || obj).length, index = dir > 0 ? 0 : length - 1; if (!initial) {// 如果没有起始值这个参数 memo = obj[keys ? keys[index] : index]; index += dir; } for (; index >= 0 && index < length; index += dir) { var currentKey = keys ? keys[index] : index; memo = iteratee(memo, obj[currentKey], currentKey, obj); } return memo; }; return function(obj, iteratee, memo, context) { var initial = arguments.length >= 3; return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial); }; }; // **Reduce** builds up a single result from a list of values, aka `inject`, // or `foldl`. _.reduce = _.foldl = _.inject = createReduce(1); // The right-associative version of reduce, also known as `foldr`. _.reduceRight = _.foldr = createReduce(-1); // Return the first value which passes a truth test. Aliased as `detect`. /* 在list中逐项查找,返回第一个通过predicate迭代函数真值检测的元素值,如果没有值传递给测试迭代器将返回 */ _.find = _.detect = function(obj, predicate, context) { var keyFinder = isArrayLike(obj) ? _.findIndex : _.findKey; var key = keyFinder(obj, predicate, context); if (key !== void 0 && key !== -1) return obj[key]; }; // Return all the elements that pass a truth test. // Aliased as `select`. /* 遍历list中的每个值,返回包含所有通过predicate真值检测的元素值 与原生的filter函数作用相同,有原生的时候可以使用原生的filter */ _.filter = _.select = function(obj, predicate, context) { var results = []; predicate = cb(predicate, context); _.each(obj, function(value, index, list) {//遍历所有元素,返回符合条件的value if (predicate(value, index, list)) results.push(value); }); return results; }; // Return all the elements for which a truth test fails. /* 返回list中没有通过predicate真值检测的元素集合,与filter相反 */ _.reject = function(obj, predicate, context) { return _.filter(obj, _.negate(cb(predicate)), context); }; // Determine whether all of the elements match a truth test. // Aliased as `all`. /* 如果list中的所有元素都通过predicate的真值检测就返回true 与原生的every函数作用相同,有原生的时候可以使用原生的every */ _.every = _.all = function(obj, predicate, context) { predicate = cb(predicate, context); var keys = !isArrayLike(obj) && _.keys(obj), length = (keys || obj).length; for (var index = 0; index < length; index++) { var currentKey = keys ? keys[index] : index; if (!predicate(obj[currentKey], currentKey, obj)) return false;//如果找到一个不符合,直接中断函数 } return true; }; // Determine if at least one element in the object matches a truth test. // Aliased as `any`. /* 如果list中有任何一个元素通过 predicate 的真值检测就返回true。一旦找到了符合条件的元素, 就直接中断对list的遍历 */ _.some = _.any = function(obj, predicate, context) { predicate = cb(predicate, context); var keys = !isArrayLike(obj) && _.keys(obj), length = (keys || obj).length; for (var index = 0; index < length; index++) { var currentKey = keys ? keys[index] : index; if (predicate(obj[currentKey], currentKey, obj)) return true; } return false; }; // Determine if the array or object contains a given item (using `===`). // Aliased as `includes` and `include`. /* 如果list包含指定的value则返回true */ _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) { if (!isArrayLike(obj)) obj = _.values(obj);//将对象中所有value压入一个数组 if (typeof fromIndex != ‘number‘ || guard) fromIndex = 0; return _.indexOf(obj, item, fromIndex) >= 0; }; // Invoke a method (with arguments) on every item in a collection. /* 在list的每个元素上执行methodName方法 */ _.invoke = restArgs(function(obj, method, args) {// 将多余三个之外的参数合并为一个数组传入到参数函数中 var isFunc = _.isFunction(method); return _.map(obj, function(value) { var func = isFunc ? method : value[method]; return func == null ? func : func.apply(value, args); }); }); // Convenience version of a common use case of `map`: fetching a property. _.pluck = function(obj, key) { return _.map(obj, _.property(key));//property() 返回获取obj[key]的函数 }; // Convenience version of a common use case of `filter`: selecting only objects // containing specific `key:value` pairs. /* 遍历list中的每一个值,返回一个数组,这个数组包含properties所列出的属性的所有的 键 - 值对 */ _.where = function(obj, attrs) { return _.filter(obj, _.matcher(attrs)); }; /* 遍历整个list,返回匹配 properties参数所列出的所有 键 - 值 对的第一个值。 */ // Convenience version of a common use case of `find`: getting the first object // containing specific `key:value` pairs. _.findWhere = function(obj, attrs) { return _.find(obj, _.matcher(attrs)); }; // Return the maximum element (or element-based computation). /* 返回list中的最大值。如果传递iteratee参数,iteratee将作为list中每个值的排序依据。如果list为空,将返回-Infinity */ _.max = function(obj, iteratee, context) { var result = -Infinity, lastComputed = -Infinity, value, computed; if (iteratee == null || (typeof iteratee == ‘number‘ && typeof obj[0] != ‘object‘) && obj != null) { // 不存在iteratee参数,或者iteratee为数字类型m,obj[0]是object类型?????????????????????????????????????????? obj = isArrayLike(obj) ? obj : _.values(obj);// 判断obj是不是数组或类数组(有没有正确类型的length) for (var i = 0, length = obj.length; i < length; i++) { value = obj[i]; if (value != null && value > result) { result = value; } } } else { // 存在iteratee参数 iteratee = cb(iteratee, context); // 利用each对整个数组进行操作 _.each(obj, function(v, index, list) { computed = iteratee(v, index, list); if (computed > lastComputed || computed === -Infinity && result === -Infinity) { result = v; lastComputed = computed; } }); } return result; }; // Return the minimum element (or element-based computation). /* 返回list中的最小值。如果传递iteratee参数,iteratee将作为list中每个值的排序依据。如果list为空,将返回-Infinity 实现与上述一致 */ _.min = function(obj, iteratee, context) { var result = Infinity, lastComputed = Infinity, value, computed; if (iteratee == null || (typeof iteratee == ‘number‘ && typeof obj[0] != ‘object‘) && obj != null) { obj = isArrayLike(obj) ? obj : _.values(obj); for (var i = 0, length = obj.length; i < length; i++) { value = obj[i]; if (value != null && value < result) { result = value; } } } else { iteratee = cb(iteratee, context); _.each(obj, function(v, index, list) { computed = iteratee(v, index, list); if (computed < lastComputed || computed === Infinity && result === Infinity) { result = v; lastComputed = computed; } }); } return result; }; // Shuffle a collection. /* 返回一个随机乱序的 list 副本, 使用 Fisher-Yates shuffle 来进行随机乱序 */ _.shuffle = function(obj) { return _.sample(obj, Infinity); }; // Sample **n** random values from a collection using the modern version of the // [Fisher-Yates shuffle](http://en.wikipedia.org/wiki/Fisher–Yates_shuffle). // If **n** is not specified, returns a single random element. // The internal `guard` argument allows it to work with `map`. /* 从 list中产生一个随机样本。传递一个数字表示从list中返回n个随机元素。否则将返回一个单一的随机项 */ _.sample = function(obj, n, guard) { if (n == null || guard) {// 没传递n或者是传递了guard参数 if (!isArrayLike(obj)) obj = _.values(obj); return obj[_.random(obj.length - 1)];// _.random()利用Math.random返回随机数组下标的值 } var sample = isArrayLike(obj) ? _.clone(obj) : _.values(obj); var length = getLength(sample); n = Math.max(Math.min(n, length), 0);// 取n和length的最小值 var last = length - 1; // 选出数量为n的随机数 // 这里不用arr.sort()方法是因为利用数组排序进行随机排列会有分布不均的现象,具体见https://www.h5jun.com/post/array-shuffle.html for (var index = 0; index < n; index++) { var rand = _.random(index, last); var temp = sample[index]; sample[index] = sample[rand]; sample[rand] = temp; } return sample.slice(0, n); }; // Sort the object‘s values by a criterion produced by an iteratee. /* 返回一个排序后的list拷贝副本。如果传递iteratee参数,iteratee将作为list中每个值的排序依据。 */ _.sortBy = function(obj, iteratee, context) { var index = 0; iteratee = cb(iteratee, context); // _.pluck()返回对象数组中所有对象的value属性组成的数组 return _.pluck(_.map(obj, function(value, key, list) {// 把数组中每个值包装成一个对象,返回一个对象数组 return { value: value, index: index++, criteria: iteratee(value, key, list) }; }).sort(function(left, right) {// 数组中的值按照函数执行结果或字符串进行排序 var a = left.criteria; var b = right.criteria; if (a !== b) { if (a > b || a === void 0) return 1; if (a < b || b === void 0) return -1; } return left.index - right.index; }), ‘value‘); }; // An internal function used for aggregate "group by" operations. /* params: 执行函数,是否分成两份???????????????????????????? */ var group = function(behavior, partition) { // 返回一个接受三个参数的函数 // 对传入数组每一项执行iteratee并将结果传入behavior执行 return function(obj, iteratee, context) { var result = partition ? [[], []] : {}; iteratee = cb(iteratee, context); _.each(obj, function(value, index) { var new_val = iteratee(value, index, obj);// 这里参数名称改为new_val感觉更好理解一点 behavior(result, value, new_val); }); return result; }; }; // Groups the object‘s values by a criterion. Pass either a string attribute // to group by, or a function that returns the criterion. /* 把一个集合分组为多个集合,通过 iterator 返回的结果进行分组. 如果 iterator 是一个字符串而不是函数, 那么将使用 iterator 作为各元素的属性名来对比进行分组 params: [[], []]或{},每一项的value与对应iteratee(value, index, obj)的结果 */ _.groupBy = group(function(result, value, key) { if (_.has(result, key)) result[key].push(value); else result[key] = [value];// 如果result中已经有了key属性,则push(value);否则新增属性key }); // Indexes the object‘s values by a criterion, similar to `groupBy`, but for // when you know that your index values will be unique. /* 给定一个list,和 一个用来返回一个在列表中的每个元素键 的iterator 函数(或属性名), 返回一个每一项索引的对象。 */ _.indexBy = group(function(result, value, key) { result[key] = value; }); // Counts instances of an object that group by a certain criterion. Pass // either a string attribute to count by, or a function that returns the // criterion. /* 排序一个列表组成一个组,并且返回各组中的对象的数量的计数。 类似groupBy,但是不是返回列表的值,而是返回在该组中值的数目。 */ _.countBy = group(function(result, value, key) { if (_.has(result, key)) result[key]++; else result[key] = 1; }); /* 第一个表示不包含代理对代码点的所有字符 第二个表示合法的代理对的所有字符 第三个表示代理对的代码点(本身不是合法的Unicode字符) 意思就是所有字符,‘hello‘.match(reStrSymbol); ==> [‘h‘,‘e‘,‘l‘,‘l‘,‘o‘] */ var reStrSymbol = /[^\ud800-\udfff]|[\ud800-\udbff][\udc00-\udfff]|[\ud800-\udfff]/g; // Safely create a real, live array from anything iterable. /* 把obj(任何可以迭代的对象)转换成一个数组,在转换 arguments 对象时非常有用 */ _.toArray = function(obj) { if (!obj) return []; if (_.isArray(obj)) return slice.call(obj);// 如果是纯数组,直接用slice.call() if (_.isString(obj)) {//如果是字符串 // Keep surrogate pair characters together return obj.match(reStrSymbol); } if (isArrayLike(obj)) return _.map(obj, _.identity);// 如果是对象类似于_.kets(obj) return _.values(obj); }; // Return the number of elements in an object. /* 返回list的长度。 */ _.size = function(obj) { if (obj == null) return 0; return isArrayLike(obj) ? obj.length : _.keys(obj).length; }; // Split a collection into two arrays: one whose elements all satisfy the given // predicate, and one whose elements all do not satisfy the predicate. /* 拆分一个数组(array)为两个数组: 第一个数组其元素都满足predicate迭代函数, 而第二个的所有元素均不能满足predicate迭代函数。 pass为传入迭代函数的返回值 */ _.partition = group(function(result, value, pass) { result[pass ? 0 : 1].push(value); }, true);
小结
1.处理类数组与对象的不同情况
var keys = !isArrayLike(obj) && _.keys(obj),//如果是数组,返回false;否则返回对象的keys数组 length = (keys || obj).length, results = Array(length);
2.高阶函数的使用
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