(If you know C# LINQ , it's like Any vs All)

• some will return true if any predicate is true

• every will return true if all predicate is true

Where predicate means function that returns bool ( true/false) for each element

every returns on first false.
some returns on first true

You're not returning the result of the element > 5 comparison; perhaps you've seen ES2015 "arrow functions" and are getting their concise syntax confused with function function syntax (or possibly you've seen some CoffeeScript).

When using function syntax, you must return the result:

var arrayForTesting = [2, 5, 18];
var result = arrayForTesting.some(function (element, index, array){
    return element > 5;
//  ^^^^^^
});

If the function exits without a return xyz statement (or uses return; with no operand), the result of calling the function is undefined, which is falsey, so some doesn't stop looping.

With ES2015, if you use an arrow function and a "concise function body," the return is implicit:

var arrayForTesting = [2, 5, 18];
var result = arrayForTesting.some(element => element > 5);

But only with concise bodies (no {}), otherwise you use return as with function functions:

var arrayForTesting = [2, 5, 18];
var result = arrayForTesting.some(element => {
    return element > 5;
});

Note that while cutting-edge browsers are actively updating their JavaScript engines to add ES2015 features, support on the ground is still sparse enough you probably need to transpile to use ES2015 features like arrow functions. (Not least if you need to support older browsers like IE10.)

See MDN:

The some() method executes the callbackFn function once for each element present in the array until it finds the one where callbackFn returns a truthy value (a value that becomes true when converted to a Boolean). If such an element is found, some() immediately returns true. Otherwise, some() returns false. callbackFn is invoked only for indexes of the array with assigned values. It is not invoked for indexes which have been deleted or which have never been assigned values.

You can see that here:

const arr = ['item 1', 'item 2', , 'item 3', , 'item 4', 'item 5'];
console.log(arr.some((item, index) => {
  console.log(item, index);
  return !item
})); 

If I will explicitly replace the empty elements with undefined, then it will return true.

That's the difference between "never assigned a value" and "been assigned the undefined value".