string to be checked for presence is a domain name (google.com) in a list of 10 million unique domain names of const type inside browser. Either I can concatenate 10million into 1 string so it is a problem of checking substring or I can use array index of to check the index.
This is similar to https://stackoverflow.com/a/1473742/2752107 and other related questions but does not address the scale.
It depends on multiple factors.
Is the array sorted in any way? If so, perhaps something like a binary search is your best bet.
Do you want to find a specific string in an unsorted array and quit the loop once you have found it? you're probably best of using Array.prototype.find() or creating a for loop and breaking as soon as you have found a string.
Do you need to check every string in the array? A simple for loop can do the trick.
Either I can concatenate 10million into 1 string so it is a problem of checking substring or I can use array index of to check the index.
Checking for a substring is slower than checking it using indices.
As Rezaa91's answer suggests, if you can order the array, you can then perform a binary search which would be way faster than a linear search.
If you do want/need to search linearly, however, use a Web Worker so your website/application remains usable and responsive while the search is in progress.
