Just make a little helper function that does what you said, loops through the array and checks each value. An efficient way to do that is with Array.some, as it will return true as soon as it finds a truthy match.

let validSLDs = ['hulu','netflix']

let string1 = 'www.hulu.com/watch/...';
let string2 = 'http://www.netflix.com/watch/...';
let string3 = 'imdb.com/title/....'

const isURLOK = (testString) => validSLDs.some(v => testString.indexOf(v) > -1);

console.log(isURLOK(string1));
console.log(isURLOK(string2));
console.log(isURLOK(string3));

The fundamental issue you are not understanding is that an Array cannot have strings as indexes. The syntax you are using is an alternate way of defining properties on to an object. All the previous suggestions people are giving you is probably confusing you more. Keep it simple. Arrays have number indexes.

// this is adding values to an array
var x = [];
x[0] = 'one';
x[1] = 'two';
console.log(x[0]);       // outputs 'one'

// this is adding a property to an object
var y = {};
y['width'] = 20;
y['height'] = 40;
console.log(y['width']); // outputs 20
console.log(y.height);   // outputs 40

// this is adding a property to our previous array 
// (because Arrays are objects too in JavaScript)
x['name'] = 'My Array';
console.log(x.name);     // outputs 'My Array'

x.indexOf('My Array');   // returns -1 because 'My Array' is not stored in the array
x.indexOf('two');        // returns 1 because that's the index of 'two' in the array