Question

['1','2','3'].map(parseInt)

返回[1, NaN, NaN]

我不知道为什么？在我看来是这样的：

['1','2','3'].map(function(i){return parseInt(i,10)})

返回[1, 2, 3]

=============================================== =======
和其他 ['1','2','3'].map(parseFloat)
返回[1, 2, 3]

Answer 1

您可以从Mozilla开发者网站找到答案：

// Consider:
["1", "2", "3"].map(parseInt);
// While one could expect [1, 2, 3]
// The actual result is [1, NaN, NaN]

// parseInt is often used with one argument, but takes two. The second being the radix
// To the callback function, Array.prototype.map passes 3 arguments: 
// the element, the index, the array
// The third argument is ignored by parseInt, but not the second one,
// hence the possible confusion. See the blog post for more details


function returnInt(element){
  return parseInt(element,10);
}

["1", "2", "3"].map(returnInt);
// Actual result is an array of numbers (as expected)

另请阅读great answer

Answer 2

查看此文章：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

回调函数指定为：

callback
Function that produces an element of the new Array, taking three arguments:
currentValue
The current element being processed in the array.
index
The index of the current element being processed in the array.
array
The array map was called upon.

因此，您的map()功能会扩展为：

parseInt('1', 0, the_array) # 1
parseInt('2', 1, the_array) # NaN
parseInt('3', 2, the_array) # NaN

什么是['1'，'2'，'3']。map（parseInt）结果

2 个答案: