（爲什麼）設置`array [NaN]`什麼也不做？

Question

我讀an article about javascript PRNGs，和我遇到的東西，讓我感到驚訝傳來：

var a = new Array(); 
var b; 
a[b++] = 1; 

a現在是[]並不會引發任何異常 - 寫入到陣列簡單地消失。如果你不相信我，請在瀏覽器控制檯中試用。

我不相信他，所以我想它在我的瀏覽器控制檯（火狐47）：

» var a = new Array(); 
» var b; 
» a[b++] = 1 

» a 
← Array [ ] 
» b 
← NaN 

有一些稀奇古怪的事情怎麼回事，但特別是，我試圖理解爲什麼陳述a[b++] = 1不[似乎]做任何事情。

Answer 1

以使其與頂部：

var a = new Array(); 
// 'a' is now an empty array, plain ol' boring empty array, could have been 
// written as a = []; 
var b; 
// 'b' have no value and is 'undefined'. 'console.log(b); // undefined' 
a[b++] = 1; 
// Lets break the above statement down into pieces: 
    b++  // Increment b by one, undefined + 1 === NaN 
a[ ]  // Use NaN as a property for our array 'a' 
     = 1; // Assign 1 to that property 
// Ok? So what happened why does 'a' still look empty? 
console.log(a); // [] 
// The way your console with show you an array is by printing the numeric keys 
// in it, and looking at the length, eg: 
// var q = []; 
// q.length = 1; 
// console.log(q); // [undefined x 1] 

// With 'a' in our case there is no numeric keys in it so [] is printed. 
// So did our value dissapear? 
// No. It is there: 
console.log('NaN' in a); // true 
// And: 
for (var prop in a) console.log(prop); // NaN 

// Why is this even a feature? 
// Arrays are extending Objects so they have the same properties as em. 
console.log(a instanceof Object); // true 
console.log(a instanceof Array); // true 

Answer 2

在那裏發生了很多事情。

1. 的代碼做了 - 它的價值1分配給a[NaN]。並且只要JS對象只能具有字符串屬性 - NaN隱含地轉換爲字符串，所以實際上您已將1分配給a["NaN"]或a.NaN。

2. console對象不是標準化的，所以你不能指望任何特定的東西。儘管FF中的當前實現迭代了數組索引。 "NaN"不是數組索引，因爲它甚至不是數字，因此控制檯中沒有顯示任何內容。

var a = new Array(); 
 
var b; 
 
a[b++] = 1; 
 

 
console.log(a[NaN], a["NaN"], a.NaN);