如果某個數組索引中存在某個值，如何檢入JavaScript？

本文翻譯自：How do I check in JavaScript if a value exists at a certain array index?

Will this work for testing whether a value at position "index" exists or not, or is there a better way: 這是否可用於測試位置“索引”的值是否存在，或者是否有更好的方法：

if(arrayName[index]==""){
     // do stuff
}

---

#1樓

參考：https://stackoom.com/question/BDCu/如果某個數組索引中存在某個值-如何檢入JavaScript

---

#2樓

Can't we just do this: 我們不能這樣做：

if(arrayName.length > 0){   
    //or **if(arrayName.length)**
    //this array is not empty 
}else{
   //this array is empty
}

---

#3樓

if(arrayName.length > index && arrayName[index] !== null) {
    //arrayName[index] has a value
}

---

#4樓

if(!arrayName[index]){
     // do stuff
}

---

#5樓

All arrays in JavaScript contain array.length elements, starting with array[0] up until array[array.length - 1] . JavaScript中的所有數組都包含array.length元素，從array[0]開始直到array[array.length - 1] 。 By definition, an array element with index i is said to be part of the array if i is between 0 and array.length - 1 inclusive. 根據定義，如果i介於0和array.length - 1之間，則具有索引i的數組元素被稱爲數組的一部分。

That is, JavaScript arrays are linear, starting with zero and going to a maximum, and arrays don't have a mechanism for excluding certain values or ranges from the array. 也就是說，JavaScript數組是線性的，從零開始並達到最大值，並且數組沒有從數組中排除某些值或範圍的機制。 To find out if a value exists at a given position index (where index is 0 or a positive integer), you literally just use 要確定給定位置索引（索引是0還是正整數）中是否存在值，您只需使用

if (index < array.length) {
  // do stuff
}

However, it is possible for some array values to be null, undefined , NaN , Infinity , 0, or a whole host of different values. 然而，可能的是一些數組值爲空， undefined ， NaN ， Infinity ，0，或不同的值的整個主機。 For example, if you add array values by increasing the array.length property, any new values will be undefined . 例如，如果通過增加array.length屬性來添加數組值，則任何新值都將是undefined 。

To determine if a given value is something meaningful, or has been defined. 確定給定值是有意義的還是已定義的。 That is, not undefined , or null : 也就是說， 沒有 undefined ，或null ：

if (typeof array[index] !== 'undefined') {

or 要麼

if (typeof array[index] !== 'undefined' && array[index] !== null) {

Interestingly, because of JavaScript's comparison rules, my last example can be optimised down to this: 有趣的是，由於JavaScript的比較規則，我的最後一個例子可以優化到這個：

if (array[index] != null) {
  // The == and != operators consider null equal to only null or undefined
}  

---

#6樓

if(typeof arr ==='object' && arr instanceof Array ){
   if(!arr.length){
      println 'empty'
   }else{
      printn 'not Empty'
   }

}else{
   println 'Null'
}

If you mean by 'Null' -> Its elements are null or equals to '' , in this case : Check if the array is empty after filtering all 'null' elements 如果你的意思是'Null' - >它的元素是null或等於''，在這種情況下：在過濾所有'null'元素後檢查數組是否爲空

if(!arr.clean().length){return 'is null'}

Of course ,Add Clean method before : 當然，之前添加Clean方法：

Array.prototype.clean=function(){return this.filter(function(e){return (typeof  e !=='undefined')&&(e!= null)&&(e!='')})}