Get to know the strange JavaScript Array.indexOf phenomenon (in Node.js)

When I try to solve ProjectEuler No.27 I came across a weird JavaScript Array.indexOf problem (in Node.js), The code looks like this:

var primes = require('./primes1000.js');
var bigPrimes = require('./primes50000.js');
var maxNumber = 2;
var maxPair = {};

var getQuadratic = function(b) {
    var i = 0;
    var a = 0;
    while (Math.abs(a) < 1000) {
        var prime = primes[i];
        var a = prime - 1 - b;
        var length = getMaxNumber(a, b) - 1;
        if (length > maxNumber) {
            maxNumber = length;
            maxPair = {
                a: a,
                b: b
            };
            console.log(maxPair, maxNumber);
        }
        i++;
    }
};

var getMaxNumber = function(a, b) {
    var isPrime = true;
    var n = 1;
    while (isPrime) {
        n = n + 1;
        var result = (n + a) * n + b;

        isPrime = (bigPrimes.indexOf(result) !== -1);
        // if ((a === 1) & (b === 41)) {
        //     console.log(result, isPrime);
        // }
    }

    return n;
};

for (var i = 0; i < primes.length; i++) {
    getQuadratic(primes[i]);
}

      

        
        
        
      

    

primes1000.js and primes50000.js are written like this:

module.exports = [2,3,5,7,11,13,17,...,997];
module.exports = [2,3,5,7,11,13,17,...,611953];

      

        
        
        
      

    

The problem arises when

bigPrimes.indexOf(result)

      

        
        
        
      

    

When the result is 47, the indexOf method returns -1, which means it was not found.

After a lot of debugging and testing, finally I changed node to

bigPrimes.indexOf(parseInt(result))

      

        
        
        
      

    

Then it works.

But the result

typeof result

      

        
        
        
      

    

and

typeof bigPrimes[14]

      

        
        
        
      

    

- both "numbers" AND

parseInt(result) === result

      

        
        
        
      

    

returns true.

I didn't understand why I need parseInt to get the correct indexOf return value.