Find given a string or number is integer

JavaScript

function isInteger(x) {
    return !(x === null) && !isNaN(x) && (x.toString().length === (new Number(x)).toFixed(0).length);
}

console.assert(isInteger(undefined) === false);
console.assert(isInteger(null) === false);
console.assert(isInteger('') === false);
console.assert(isInteger('') === false);
console.assert(isInteger(100) === true);
console.assert(isInteger(100.33) === false);
console.assert(isInteger('100.33') === false);
console.assert(isInteger('100') === true);
console.assert(isInteger("JOHN") === false);
console.assert(isInteger("100JOHN") === false);
console.assert(isInteger("JOHN100") === false);