javascript - 传递的参数 'guard' 在 underscore.js 函数中检查什么？

Question

Get the first element of an array. Passing n will return the first N values in the array. Aliased as head and take. The guard check allows it to work with _.map.

  _.first = _.head = _.take = function(array, n, guard) {
    if (array == null) return void 0;
    return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
  };

在这个 underscore.js 函数中变量 'guard' 有什么用？

Answer 1

如果您查看源代码:

  // Get the first element of an array. Passing **n** will return the first N
  // values in the array. Aliased as `head` and `take`. The **guard** check
  // allows it to work with `_.map`.
  _.first = _.head = _.take = function(array, n, guard) {
    if (array == null) return void 0;
    return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
  };

The guard check allows it to work with _.map.

所以如果你有这样一个数组:

var a = [ [1, 2, 3], [4, 5, 6] ];
// put this array though _.map and _.first
_.map(a, _.first); // [1, 4]

如果不是这种情况，您的结果将如下所示:

[ [], [4] ]

因为进入 _.map 的参数:

_.map(['a', 'b', 'c'], function(val, key, obj) {
    // key = 0, 1, 2
    // val = a, b, c
    // obj = ['a', 'b', 'c'] 
    // the obj argument is why `guard` is truly and the first element in the array is returned rater than using [].slice
});

---

它不漂亮，但它允许一起工作:

_.first([1, 2, 3], 2) // [1, 2]
_.first([1, 2, 3], 2, true) // 1
_.first([1, 2, 3], 2, 3) // 1