I’m using JavaScript Array Every method for some validation and when I’m passing arguments with some condition, then it’s not working with JavaScript lambda callbackFn.

Let me describe the issue with simple example.

According to Array.prototype.every() documentation, it’s syntax is:

every(callbackFn)
every(callbackFn, thisArg)

In this example I’m validating, all array elements are even or odd.

When I write array.every() method callbackFn as a normal JavaScript function, then validation is working.
This is example as follows:

const checkEven = function (num)  {
   console.log(this.isEven, num);
   return num%2 === (this?.isEven ? 0 : 1);
}

evenArrary = [2, 4, 6, 8];
console.log('All elements are even: ', evenArrary.every(checkEven, {isEven: true}));

oddArray   = [1, 3, 5, 7];
console.log('All elements are odd: ', oddArray.every(checkEven, {isEven: false}));

Now, I’m changing that callbackFn as a JavaScript lambda function, then passing argument is not working.
Example as follows:

const checkEven = ((num) => {
   // 'this' is empty object here & this.isEven = undefined
   console.log(this.isEven, num);
   return num%2 === (this?.isEven ? 0 : 1);
});

evenArrary = [2, 4, 6, 8];
console.log('All elements are even: ', evenArrary.every(checkEven, {isEven: true}));

oddArray   = [1, 3, 5, 7];
console.log('All elements are odd: ', oddArray.every(checkEven, {isEven: false}));

Output:

Now, I don’t understand why passing arguments is not working when write callbackFn as a lambda function.

Why this is happening ?