The same as in JavaScript, using Array.prototype.indexOf():

console.log(channelArray.indexOf('three') > -1);

Or using ECMAScript 2016 Array.prototype.includes():

console.log(channelArray.includes('three'));

Note that you could also use methods like showed by @Nitzan to find a string. However you wouldn't usually do that for a string array, but rather for an array of objects. There those methods were more sensible. For example

const arr = [{foo: 'bar'}, {foo: 'bar'}, {foo: 'baz'}];
console.log(arr.find(e => e.foo === 'bar')); // {foo: 'bar'} (first match)
console.log(arr.some(e => e.foo === 'bar')); // true
console.log(arr.filter(e => e.foo === 'bar')); // [{foo: 'bar'}, {foo: 'bar'}]

Reference

Array.find()

Array.some()

Array.filter()

You can use the some method:

console.log(channelArray.some(x => x === "three")); // true

You can use the find method:

console.log(channelArray.find(x => x === "three")); // three

Or you can use the indexOf method:

console.log(channelArray.indexOf("three")); // 2

If your code is ES7 based (or upper versions):

channelArray.includes('three'); //will return true or false

If not, for example you are using IE with no babel transpile:

channelArray.indexOf('three') !== -1; //will return true or false

the indexOf method will return the position the element has into the array, because of that we use !== different from -1 if the needle is found at the first position.