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Why won't my JavaScript account system code work or run? [duplicate]

This question already has answers here:
How do I check if an array includes a value in JavaScript?
(60 answers)
Closed 6 years ago.
Is there an easier way to determine if a variable is equal to a range of values, such as:
if x === 5 || 6
rather than something obtuse like:
if x === 5 || x === 6
?

You can stash your values inside an array and check whether the variable exists in the array by using [].indexOf:
if([5, 6].indexOf(x) > -1) {
// ...
}
If -1 is returned then the variable doesn't exist in the array.

Depends on what sort of test you're performing. If you've got static strings, this is very easy to check via regular expressions:
if (/^[56ab]$/.test(item)) {
//-or-
if (/^(foo|bar|baz|fizz|buzz)$/.test(item)) {
doStuff();
} else {
doOtherStuff();
}
If you've got a small set of values (string or number), you can use a switch:
switch (item) {
case 1:
case 2:
case 3:
doStuff();
break;
default:
doOtherStuff();
break;
}
If you've got a long list of values, you should probably use an array with ~arr.indexOf(item), or arr.contains(item):
vals = [1,3,18,3902,...];
if (~vals.indexOf(item)) {
doStuff();
} else {
doOtherStuff();
}
Unfortunately Array.prototype.indexOf isn't supported in some browsers. Fortunately a polyfill is available. If you're going through the trouble of polyfilling Array.prototype.indexOf, you might as well add Array.prototype.contains.
Depending on how you're associating data, you could store a dynamic list of strings within an object as a map to other relevant information:
var map = {
foo: bar,
fizz: buzz
}
if (item in map) {
//-or-
if (map.hasOwnProperty(item)) {
doStuff(map[item]);
} else {
doOtherStuff();
}
in will check the entire prototype chain while Object.prototype.hasOwnProperty will only check the object, so be aware that they are different.

It's perfectly fine. If you have a longer list of values, perhaps you can use the following instead:
if ([5,6,7,8].indexOf(x) > -1) {
}

Yes. You can use your own function. This example uses .some:
var foo = [ 5, 6 ].some(function(val) {
return val === x;
});
foo; // true

This is what I've decided to use:
Object.prototype.isin = function() {
for(var i = arguments.length; i--;) {
var a = arguments[i];
if(a.constructor === Array) {
for(var j = a.length; j--;)
if(a[j] == this) return true;
}
else if(a == this) return true;
}
return false;
}
You would use it like this:
var fav = 'pear',
fruit = ['apple', 'banana', 'orange', 'pear'],
plu = [4152, 4231, 3030, 4409];
if (fav.isin(fruit, plu, 'eggs', 'cheese')) {
//do something cool
}
The advantages are:
it works in IE < 9;
it reads naturally from left to right;
you can feed it arrays or separate values.
If you don't want to allow type coercion (indexOf does not), change the two == to ===. As it stands:
fav = "4231";
plu.indexOf(fav) //-1
fav.isin(plu) //true

no, there might be a few tricks that are case specific but in general i write code like this:
if (someVariable === 1 ||
someVariable === 2 ||
someVariable === 7 ||
someVariable === 12 ||
someVariable === 14 ||
someVariable === 19) {
doStuff();
moreStuff();
} else {
differentStuff();
}

The simple answer is no. You can use a switch statement, which is easier to read if you are comparing a lot of string values, but using it for two values wouldn't look any better.

[Edit] this seems to work, but as Dan pointed out, it is actually a false positive. Do not use this method. I leave it here for educational purposes.
Easiest way I know :
a = [1,2,3,4,5];
if(3 in a) alert("true"); // will alert true
Tested in Chrome console. Not sure if it works in other browsers.

How do I recognize an empty JSON object in JS returned by my Rails controller? [duplicate]

This question already has answers here:
How do I test for an empty JavaScript object?
(48 answers)
Closed 4 years ago.
How do I check for an empty JSON object in Javascript? I thought the key was
var isEmpty = (response || []).length === 0;
but this isn't it. In my JS console, I have tried
jsonresp = {}
{}
jsonresp.length
undefined
What's driving this is I have a Ruby on Rails controller that is returning the following
render json: job_id.eql?(cur_job_id) ? {} : json_obj
In the case where the controller returns the "{}" is where I'm having trouble on the JS side recognizing if it is empty or not.

You can also try:
if (typeof jsonresp == 'undefined' || typeof jsonresp == 'null'){
// some logic here
}

You can use Object.keys() function to get keys of the object as an array and then check the array to see whether it is empty.
var obj = {};
var arr = Object.keys(obj);
console.log(arr);
console.log("Is Empty : " + (arr.length == 0));

🏄 Shortly:
const isEmpty = !Object.keys(jsonresp).length

How to detect that a multi-level object has an undefined or null property? [duplicate]

This question already has answers here:
Test for existence of nested JavaScript object key
(64 answers)
Closed 6 years ago.
In many places in my code, I have checks similar to the one below. It's very verbose, and ugly. Is there is better way? FYI, I'm using Lodash in all my projects, so I have access to that powerful library.
if (myAssessments[orderId].report &&
myAssessments[orderId].report[categoryProductCode] &&
myAssessments[orderId].report[categoryProductCode].categories &&
myAssessments[orderId].report[categoryProductCode].categories[comment.categoryId]) {
// Do something related to
// myAssessments[orderId].report[categoryProductCode].categories[comment.categoryId]
}

Since you use lodash, you might use the has method:
_.has(obj,[orderId, 'report', categoryProductCode, 'categories', comment.categoryId])
https://lodash.com/docs/4.16.6#has
Or the get method to get the value of the object path: https://lodash.com/docs/4.16.6#get

Not elegant way but you can wrap in try catch
var result;
try{
result = myAssessments[orderId].report[categoryProductCode].categories[comment.categoryId]
}catch{}
if (result){
// do it
}

Use the built-in isset function:
if (isset(myAssessments[orderId].report) &&
isset(myAssessments[orderId].report[categoryProductCode]) &&
isset(myAssessments[orderId].report[categoryProductCode].categories) &&
isset(myAssessments[orderId].report[categoryProductCode].categories[comment.categoryId)]) {

You could use an array with all properties to check and iterate until all properties have been checked.
function checkProperties(object, keys) {
return keys.every(function (key) {
if (key in object) {
object = object[key];
return true;
}
});
}
// usage
if (checkProperties(myAssessments, [orderId, 'report', categoryProductCode, 'categories', comment.categoryId])) {
// Do something related to
// myAssessments[orderId].report[categoryProductCode].categories[comment.categoryId]
}

I have this genric function
function chckForKeyPresence(data, arr, checkLength){
var currData = data;
for(var i=0; i<arr.length; i++){
if(!currData.hasOwnProperty(arr[i]))
return false;
currData = currData[arr[i]];
}
if(checkLength)
if(currData.length==0)
return false;
return true;
}
Here 1st argument is the main data, 2nd argument is the array of properties you need to check and the third argument will check the length of the last element that it is 0 or not, it will check only if the third argument is true.
You can use it like:
if(!chckForKeyPresence(data, ["results", "tweets"], true)){
// error
return;
}

How to check - $scope.students = []; - in JavaScript - Angular JS [duplicate]

When the page is loading for the first time, I need to check if there is an image in image_array and load the last image.
Otherwise, I disable the preview buttons, alert the user to push new image button and create an empty array to put the images;
The problem is that image_array in the else fires all time. If an array exists - it just overrides it, but alert doesn't work.
if(image_array.length > 0)
$('#images').append('<img src="'+image_array[image_array.length-1]+'" class="images" id="1" />');
else{
$('#prev_image').attr('disabled', 'true');
$('#next_image').attr('disabled', 'true');
alert('Please get new image');
var image_array = [];
}
UPDATE
Before loading html, I have something like this:
<?php if(count($images) != 0): ?>
<script type="text/javascript">
<?php echo "image_array = ".json_encode($images);?>
</script>
<?php endif; ?>

if (typeof image_array !== 'undefined' && image_array.length > 0) {
// the array is defined and has at least one element
}
Your problem may be happening due to a mix of implicit global variables and variable hoisting. Make sure you use var whenever declaring a variable:
<?php echo "var image_array = ".json_encode($images);?>
// add var ^^^ here
And then make sure you never accidently redeclare that variable later:
else {
...
image_array = []; // no var here
}

To check if an array is either empty or not
A modern way, ES5+:
if (Array.isArray(array) && array.length) {
// array exists and is not empty
}
An old-school way:
typeof array != "undefined"
&& array != null
&& array.length != null
&& array.length > 0
A compact way:
if (typeof array != "undefined" && array != null && array.length != null && array.length > 0) {
// array exists and is not empty
}
A CoffeeScript way:
if array?.length > 0
Why?
Case Undefined
Undefined variable is a variable that you haven't assigned anything to it yet.
let array = new Array(); // "array" !== "array"
typeof array == "undefined"; // => true
Case Null
Generally speaking, null is state of lacking a value. For example a variable is null when you missed or failed to retrieve some data.
array = searchData(); // can't find anything
array == null; // => true
Case Not an Array
Javascript has a dynamic type system. This means we can't guarantee what type of object a variable holds. There is a chance that we're not talking to an instance of Array.
supposedToBeArray = new SomeObject();
typeof supposedToBeArray.length; // => "undefined"
array = new Array();
typeof array.length; // => "number"
Case Empty Array
Now since we tested all other possibilities, we're talking to an instance of Array. In order to make sure it's not empty, we ask about number of elements it's holding, and making sure it has more than zero elements.
firstArray = [];
firstArray.length > 0; // => false
secondArray = [1,2,3];
secondArray.length > 0; // => true

How about (ECMA 5.1):
if(Array.isArray(image_array) && image_array.length){
// array exists and is not empty
}

This is what I use. The first condition covers truthy, which has both null and undefined. Second condition checks for an empty array.
if(arrayName && arrayName.length > 0){
//do something.
}
or thanks to tsemer's comment I added a second version
if(arrayName && arrayName.length)
Then I made a test for the second condition, using Scratchpad in Firefox:
var array1;
var array2 = [];
var array3 = ["one", "two", "three"];
var array4 = null;
console.log(array1);
console.log(array2);
console.log(array3);
console.log(array4);
if (array1 && array1.length) {
console.log("array1! has a value!");
}
if (array2 && array2.length) {
console.log("array2! has a value!");
}
if (array3 && array3.length) {
console.log("array3! has a value!");
}
if (array4 && array4.length) {
console.log("array4! has a value!");
}
which also proves that if(array2 && array2.length) and if(array2 && array2.length > 0) are exactly doing the same

optional chaining
As optional chaining proposal reached stage 4 and is getting wider support, there is a very elegant way to do this
if(image_array?.length){
// image_array is defined and has at least one element
}

You should use:
if (image_array !== undefined && image_array.length > 0)

If you want to test whether the image array variable had been defined you can do it like this
if(typeof image_array === 'undefined') {
// it is not defined yet
} else if (image_array.length > 0) {
// you have a greater than zero length array
}

JavaScript
( typeof(myArray) !== 'undefined' && Array.isArray(myArray) && myArray.length > 0 )
Lodash & Underscore
( _.isArray(myArray) && myArray.length > 0 )

You can use jQuery's isEmptyObject() to check whether the array contains elements or not.
var testArray=[1,2,3,4,5];
var testArray1=[];
console.log(jQuery.isEmptyObject(testArray)); //false
console.log(jQuery.isEmptyObject(testArray1)); //true
Source: https://api.jquery.com/jQuery.isEmptyObject/

Using undescore or lodash:
_.isArray(image_array) && !_.isEmpty(image_array)

A simple way that doesn't result in exceptions if not exist and convert to boolean:
!!array
Example:
if (!!arr) {
// array exists
}

How about this ? checking for length of undefined array may throw exception.
if(image_array){
//array exists
if(image_array.length){
//array has length greater than zero
}
}

The best is to check like:
let someArray: string[] = [];
let hasAny1: boolean = !!someArray && !!someArray.length;
let hasAny2: boolean = !!someArray && someArray.length > 0; //or like this
console.log("And now on empty......", hasAny1, hasAny2);
See full samples list:

I come across this issue quite a lot in Javascript. For me the best way to do it is to put a very broad check before checking for length. I saw some other solutions in this Q&A, but I wanted to be able to check for either null or undefined or any other false value.
if(!array || array.length == 0){
console.log("Array is either empty or does not exist")
}
This will first check for undefined, null, or other false values. If any of those are true, it will complete the boolean as this is an OR. Then the more risky check of array.length, which could error us if array is undefined, can be checked. This will never be reached if array is undefined or null, so the ordering of conditions is very important.

If you do not have a variable declared as array you can create a check:
if(x && x.constructor==Array && x.length){
console.log("is array and filed");
}else{
var x= [];
console.log('x = empty array');
}
This checks if variable x exists and if it is, checks if it is a filled array. else it creates an empty array (or you can do other stuff);
If you are certain there is an array variable created there is a simple check:
var x = [];
if(!x.length){
console.log('empty');
} else {
console.log('full');
}
You can check my fiddle here with shows most possible ways to check array.

The following is my solution wrapped in a function that also throws
errors to manage a couple of problems with object scope and all types
of possible data types passed to the function.
Here's my fiddle used to examine this problem (source)
var jill = [0];
var jack;
//"Uncaught ReferenceError: jack is not defined"
//if (typeof jack === 'undefined' || jack === null) {
//if (jack) {
//if (jack in window) {
//if (window.hasOwnP=roperty('jack')){
//if (jack in window){
function isemptyArray (arraynamed){
//cam also check argument length
if (arguments.length === 0) {
throw "No argument supplied";
}
//console.log(arguments.length, "number of arguments found");
if (typeof arraynamed !== "undefined" && arraynamed !== null) {
//console.log("found arraynamed has a value");
if ((arraynamed instanceof Array) === true){
//console.log("I'm an array");
if (arraynamed.length === 0) {
//console.log ("I'm empty");
return true;
} else {
return false;
}//end length check
} else {
//bad type
throw "Argument is not an array";
} //end type check
} else {
//bad argument
throw "Argument is invalid, check initialization";;
}//end argument check
}
try {
console.log(isemptyArray(jill));
} catch (e) {
console.log ("error caught:",e);
}

the way I found to work (comming from another language) is to make a simple function to test.
create a function that check the size of the array and pass the lenght by parameter.
isEmpty(size){
if(size==0) {
return true;
} else {
return false;
}
}
//then check
if(isEmpty(yourArray.length)==true){
//its empty
} else {
//not empty
}

You should do this
if (!image_array) {
// image_array defined but not assigned automatically coerces to false
} else if (!(0 in image_array)) {
// empty array
// doSomething
}

For me sure some of the high rated answers "work" when I put them into jsfiddle, but when I have a dynamically generated amount of array list a lot of this code in the answers just doesn't work for ME.
This is what IS working for me.
var from = [];
if(typeof from[0] !== undefined) {
//...
}
Notice, NO quotes around undefined and I'm not bothering with the length.

Probably your image_array is not array but some OBJECT with length property (like string) - try
if(image_array instanceof Array && image_array.length)
function test(image_array) {
if(image_array instanceof Array && image_array.length) {
console.log(image_array,'- it is not empty array!')
} else {
console.log(image_array,'- it is empty array or not array at all!')
}
}
test({length:5});
test('undefined');
test([]);
test(["abc"]);

In my case, array_.length always returned 0, even if it had values inside. Probably, because of non-default indexes.
So to check if array is defined we use typeof _array !== 'undefined'
And then to check if it contains any date i just simply compare it to an empty array _array !== []

in ts
isArray(obj: any)
{
return Array.isArray(obj)
}
in html
(photos == undefined || !(isArray(photos) && photos.length > 0) )

When you create your image_array, it's empty, therefore your image_array.length is 0
As stated in the comment below, i edit my answer based on this question's answer) :
var image_array = []
inside the else brackets doesn't change anything to the image_array defined before in the code

Javascript Checking Variable [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How do I test for an empty Javascript object from JSON?
var test= {};
var incidentReport = {
"place1": "n/a",
"place2": "n/a",
"place3": "n/a",
}
Above are the two ways my varible is going to look. Ive tryed doing the following code to test if its empty/looks like {}
if(test == "")
and tried
if(test == null)
also tried
if(!test)
Does anyone know where I am going wrong? Just a beginner to JavaScript and JSON. Is what I am doing considered back practice are there better ways to declare this empty?
Thanks for the support

Use JSON.stringify
var test= {};
if(JSON.stringify(test).length==2)
alert('null')

if(test == "")
checks if it is an empty string, so this won't work
if(test == null)
checks if it is null which is "similar" to undefined - this isn't the case
if(!test)
checks if it is a falsy value, this in not the case either.
You have to check if there exist child-elements (properties):
function isEmpty(obj) {
for(var prop in obj) {
if(obj.hasOwnProperty(prop)) return false;
}
return true;
}
if ( isEmpty(test) ){...}
The very important point is the .hasOwnProperty() - this checks if it is a real property of the object and not only inherited through the prototype chain.

test here is an object. so you have to check if there are any prioperties/elements int his object. You can try something like below
var test= {};
function isEmptyObject(obj) {
// This works for arrays too.
for(var name in obj) {
return false
}
return true
}
alert("is this object empty?" + isEmptyObject(test));​