Returning true if JavaScript array contains an element

You could just use Array.prototype.some

var isMatch = viewedUserLikedUsersArray.some(function(user){
  return user === logged_in_user;
});

It stops when it finds a true value

Probably going to get blasted for some reason but hey, why not!

function findMatch(arr, user) {
    var i = 0, count = arr.length, matchFound = false;

    for(; i < count; i++) {
        if (arr[i] === user) {
            matchFound = true;
            break;
        }
    }

    return matchFound;
}

 var isMatch = findMatch(viewedUserLikedUsersArray, logged_in_user); // etc.

An alternative could also be to use includes()

var isMatch = viewedUserLikedUsersArray.includes(logged_in_user); // returns true/false

The One and Only ChemistryBlob answer with Array.prototype

Array.prototype.findMatch = function(token) {
    var i = 0, count = this.length, matchFound = false;

    for(; i < count; i++) {
        if (this[i] === token) {
            matchFound = true;
            break;
        }
    }

    return matchFound;
}

var isMatch = viewedUserLikedUsersArray.findMatch(logged_in_user); // etc.

Use some as it allows the browser to stop as soon as one element is found that matches, so it's going to be faster:

const newArr = Array.some((e) => e.name === "amboji")
console.log(newArr);  // returns true if it contains, else returns false

At first, we have to understand, what is stored in viewedUserLikedUsersArray array.

If there are primitives, it's ok, but if there are objects we can't use indexOf method of array, because it use strict comparison === inside, how we know, the comparison with object is going by link.

indexOf works similar to iterating through the loop, the only way.

In case with objects we can use find method of array MDN Array.prototype.find() or findIndex Array.prototype.findIndex();

Also we can store users in hashMap with userId keys, and check matching by referring to the object property.

var someUsers = {
  '#124152342': {
    ...
  },
  '#534524235': {
    ...
  },
  ...
};

...

var someUserId = '#124152342';

if (someUsers[someUserId]) {
  console.log('is match');
} else {
  console.log('no match');
}