是什么[ '1'， '2'， '3'。图（parseInt函数）导致

Question

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

回报[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)}) 

回报[1, 2, 3]

============================== ========================
等
['1','2','3'].map(parseFloat)
回报[1, 2, 3]

Answer 1

看看这篇文章：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

指定的回调函数为：

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map()功能扩展为：

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

Answer 2

您可以从Mozilla的开发网站找到答案：

// 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) 

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又读great answer