如何检查javascript中是否存在数组索引？

Question

I am working with Titanium, my code looks like this:

var currentData = new Array();
if(currentData[index]!==""||currentData[index]!==null||currentData[index]!=='null')
{
    Ti.API.info("is exists  " + currentData[index]);
    return true;
}
else
{   
    return false;
}

I am passing an index to the currentData array. I still can't detect a non-existing index using the above code.

Answer 1

Use typeof arrayName[index] === 'undefined'

ie

if(typeof arrayName[index] === 'undefined') {
    // does not exist
}
else {
    // does exist
}

Answer 2

var myArray = ["Banana", "Orange", "Apple", "Mango"];

if (myArray.indexOf(searchTerm) === -1) {
  console.log("element doesn't exist");
}
else {
  console.log("element found");
}

Answer 3

Someone please correct me if i'm wrong, but AFAIK the following is true:

1. Arrays are really just Objects under the hood of JS
2. Thus, they have the prototype method hasOwnProperty "inherited" from Object
3. in my testing, hasOwnProperty can check if anything exists at an array index.

So, as long as the above is true, you can simply:

const arrayHasIndex = (array, index) => Array.isArray(array) && array.hasOwnProperty(index);

usage:

arrayHasIndex([1,2,3,4],4); outputs: false

arrayHasIndex([1,2,3,4],2); outputs: true

Answer 4

This is exactly what the in operator is for. Use it like this:

if (index in currentData) 
{ 
    Ti.API.info(index + " exists: " + currentData[index]);
}

The accepted answer is wrong, it will give a false negative if the value at index is undefined :

 const currentData = ['a', undefined], index = 1; if (index in currentData) { console.info('exists'); } // ...vs... if (typeof currentData[index] !== 'undefined') { console.info('exists'); } else { console.info('does not exist'); // incorrect! }

Answer 5

这些天我会利用 ecmascript 并像那样使用它

return myArr?.[index]

Answer 6

I had to wrap techfoobar's answer in a try .. catch block, like so:

try {
  if(typeof arrayName[index] == 'undefined') {
    // does not exist
  }
  else {
  // does exist
  }
} 
catch (error){ /* ignore */ }

...that's how it worked in chrome, anyway (otherwise, the code stopped with an error).

Answer 7

If elements of array are also simple objects or arrays, you can use some function:

// search object
var element = { item:'book', title:'javasrcipt'};

[{ item:'handbook', title:'c++'}, { item:'book', title:'javasrcipt'}].some(function(el){
    if( el.item === element.item && el.title === element.title ){
        return true; 
     } 
});

[['handbook', 'c++'], ['book', 'javasrcipt']].some(function(el){
    if(el[0] == element.item && el[1] == element.title){
        return true;
    }
});

Answer 8

Consider the array a:

var a ={'name1':1, 'name2':2}

If you want to check if 'name1' exists in a, simply test it with in :

if('name1' in a){
console.log('name1 exists in a')
}else
console.log('name1 is not in a')

Answer 9

var demoArray = ['A','B','C','D'];
var ArrayIndexValue = 2;
if(ArrayIndexValue in demoArray){
   //Array index exists
}else{
   //Array Index does not Exists
}

Answer 10

If you are looking for some thing like this.

Here is the following snippetr

 var demoArray = ['A','B','C','D']; var ArrayIndexValue = 2; if(demoArray.includes(ArrayIndexValue)){ alert("value exists"); //Array index exists }else{ alert("does not exist"); //Array Index does not Exists }

Answer 11

 var fruits = ["Banana", "Orange", "Apple", "Mango"]; if(fruits.indexOf("Banana") == -1){ console.log('item not exist') } else { console.log('item exist') }

Answer 12

If you use underscore.js then these type of null and undefined check are hidden by the library.

So your code will look like this -

var currentData = new Array();

if (_.isEmpty(currentData)) return false;

Ti.API.info("is exists  " + currentData[index]);

return true;

It looks much more readable now.

Answer 13

This way is easiest one in my opinion.

var nameList = new Array('item1','item2','item3','item4');

// Using for loop to loop through each item to check if item exist.

for (var i = 0; i < nameList.length; i++) {
if (nameList[i] === 'item1') 
{   
   alert('Value exist');
}else{
   alert('Value doesn\'t exist');
}

And Maybe Another way to do it is.

nameList.forEach(function(ItemList)
 {
   if(ItemList.name == 'item1')
        {
          alert('Item Exist');
        }
 }

Answer 14

Simple way to check item exist or not

Array.prototype.contains = function(obj) {
    var i = this.length;
    while (i--)
       if (this[i] == obj)
       return true;
    return false;
}

var myArray= ["Banana", "Orange", "Apple", "Mango"];

myArray.contains("Apple")

Answer 15

When trying to find out if an array index exists in JS, the easiest and shortest way to do it is through double negation.

let a = [];
a[1] = 'foo';
console.log(!!a[0])   // false
console.log(!!a[1])   // true

Answer 16

const arr = []

typeof arr[0] // "undefined"

arr[0] // undefined

If boolean expression

typeof arr[0] !== typeof undefined

is true then 0 is contained in arr

Answer 17

One line validation. The simplest way.

return !!currentData[index];

Outputs

var testArray = ["a","b","c"]

testArray[5]; //output => undefined
testArray[1]; //output => "b"

!!testArray[5]; //output => false
!!testArray[1]; //output => true

Answer 18

you can simply use this:

var tmp = ['a', 'b'];
index = 3 ;
if( tmp[index]){
    console.log(tmp[index] + '\n');
}else{
    console.log(' does not exist');
}

Answer 19

(typeof files[1] === undefined)?
            this.props.upload({file: files}):
            this.props.postMultipleUpload({file: files widgetIndex: 0, id})

Check if the second item in the array is undefined using the typeof and checking for undefined

Answer 20

if(typeof arrayName[index] == undefined) {
    console.log("Doesn't exist")
}
else {
console.log("does exist")
}

Answer 21

This also works fine, testing by type against undefined .

if (currentData[index] === undefined){return}

Test:

 const fruits = ["Banana", "Orange", "Apple", "Mango"]; if (fruits["Raspberry"] === undefined){ console.log("No Raspberry entry in fruits!") }