How do I check in JavaScript if a value exists at a certain array index?
这是否会用于测试位置“index”处的值是否存在，还是有更好的方法：
if(arrayName[index]==""){
// do stuff
}
All arrays in JavaScript contain array.length elements, starting with array[0] up until 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.
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. To find out if a value exists at a given position index (where index is 0 or a positive integer), you literally just use
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. For example, if you add array values by increasing the array.length property, any new values will be undefined .
To determine if a given value is something meaningful, or has been defined. That is, not undefined , or null :
if (typeof array[index] !== 'undefined') {
or
if (typeof array[index] !== 'undefined' && array[index] !== null) {
Interestingly, because of JavaScript's comparison rules, my last example can be optimised down to:
if (array[index] != null) {
// The == and != operator consider null equal to only null or undefined
我们不能这样做：
if(arrayName.length > 0){
//or **if(arrayName.length)**
//this array is not empty
}else{
//this array is empty
}
Using only .length is not safe and will cause an error in some browsers. Here is a better solution:
if(array && array.length){
// not empty
} else {
// empty
}
or, we can use:
Object.keys(__array__).length
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