How to Determine If an Array Has Any Elements or Not

How to determine if an array has any elements or not?

You can use the count() or sizeof() PHP functions:

if (sizeof($result) > 0) {
    echo "array size is greater than zero";
}
else {
    echo "array size is zero";
}

Or you can use:

if (count($result) > 0) {
    echo "array size is greater than zero";
}
else {
    echo "array size is zero";
}

Check if an element is present in an array

ECMAScript 2016 incorporates an includes() method for arrays that specifically solves the problem, and so is now the preferred method.

[1, 2, 3].includes(2);     // true
[1, 2, 3].includes(4);     // false
[1, 2, 3].includes(1, 2);  // false (second parameter is the index position in this array at which to begin searching)

As of JULY 2018, this has been implemented in almost all major browsers, if you need to support an older browser a polyfill is available.

Edit: Note that this returns false if the item in the array is an object. This is because similar objects are two different objects in JavaScript.

How do I check if an array includes a value in JavaScript?

Modern browsers have Array#includes, which does exactly that and is widely supported by everyone except IE:

console.log(['joe', 'jane', 'mary'].includes('jane')); //true

Determine whether an array contains a value

var contains = function(needle) {
    // Per spec, the way to identify NaN is that it is not equal to itself
    var findNaN = needle !== needle;
    var indexOf;

    if(!findNaN && typeof Array.prototype.indexOf === 'function') {
        indexOf = Array.prototype.indexOf;
    } else {
        indexOf = function(needle) {
            var i = -1, index = -1;

            for(i = 0; i < this.length; i++) {
                var item = this[i];

                if((findNaN && item !== item) || item === needle) {
                    index = i;
                    break;
                }
            }

            return index;
        };
    }

    return indexOf.call(this, needle) > -1;
};

You can use it like this:

var myArray = [0,1,2],
    needle = 1,
    index = contains.call(myArray, needle); // true

CodePen validation/usage

How to check if array is empty or does not exist?

You want to do the check for undefined first. If you do it the other way round, it will generate an error if the array is undefined.

if (array === undefined || array.length == 0) {
    // array does not exist or is empty
}

Update

This answer is getting a fair amount of attention, so I'd like to point out that my original answer, more than anything else, addressed the wrong order of the conditions being evaluated in the question. In this sense, it fails to address several scenarios, such as null values, other types of objects with a length property, etc. It is also not very idiomatic JavaScript.

The foolproof approach

Taking some inspiration from the comments, below is what I currently consider to be the foolproof way to check whether an array is empty or does not exist. It also takes into account that the variable might not refer to an array, but to some other type of object with a length property.

if (!Array.isArray(array) || !array.length) {
  // array does not exist, is not an array, or is empty
  // ⇒ do not attempt to process array
}

To break it down:

1. Array.isArray(), unsurprisingly, checks whether its argument is an array. This weeds out values like null, undefined and anything else that is not an array.
   
   Note that this will also eliminate array-like objects, such as the arguments object and DOM NodeList objects. Depending on your situation, this might not be the behavior you're after.

2. The array.length condition checks whether the variable's length property evaluates to a truthy value. Because the previous condition already established that we are indeed dealing with an array, more strict comparisons like array.length != 0 or array.length !== 0 are not required here.

The pragmatic approach

In a lot of cases, the above might seem like overkill. Maybe you're using a higher order language like TypeScript that does most of the type-checking for you at compile-time, or you really don't care whether the object is actually an array, or just array-like.

In those cases, I tend to go for the following, more idiomatic JavaScript:

if (!array || !array.length) {
    // array or array.length are falsy
    // ⇒ do not attempt to process array
}

Or, more frequently, its inverse:

if (array && array.length) {
    // array and array.length are truthy
    // ⇒ probably OK to process array
}

With the introduction of the optional chaining operator (Elvis operator) in ECMAScript 2020, this can be shortened even further:

if (!array?.length) {
    // array or array.length are falsy
    // ⇒ do not attempt to process array
}

Or the opposite:

if (array?.length) {
    // array and array.length are truthy
    // ⇒ probably OK to process array
}

Check the array has empty element or not

As of ES2016, you should use Array.prototype.includes:

const array = ["a", "b", , "d"];
array.includes(undefined); // true

(You don't need to write undefined, but this makes it more clear what's happening.)

Note that this method treats slots valued undefined and empty slots the same, although they're not. If you need to differentiate these two cases as well, starting from ES2017, you can use Object.values making the following expression true if there are empty slots in the array:

Object.values(array).length !== array.length; // true

Check if a string contains any element of an array in JavaScript

Problem lies in the for loop, which only iterates once since return ends the function, cutting off the for loop in the process. So, you can update the code like so to make the function only return once the for loop has been completed .

var arr = ['banana', 'monkey banana', 'apple', 'kiwi', 'orange'];
function checker(value) {  var prohibited = ['banana', 'apple'];
  for (var i = 0; i < prohibited.length; i++) {    if (value.indexOf(prohibited[i]) > -1) {      return false;    }  }  return true;}
arr = arr.filter(checker);console.log(arr);

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