jQuery or JavaScript equivalent of PHP strpos function to find string on a page

I assume you mean check whether a string contains a character, and the position in the string – you’d like to use the indexOf() method of a string in JS. Here are the relevant docs.

Okay, so you’d like to search the whole page! The :contains() selector will do that. See the jQuery docs for :contains.

To search every element in the page, use

var has_string = $('*:contains("search text")');

If you get jQuery elements back, then the search was a success. For example, on this very page

var has_string=$('*:contains("Alex JL")').length
//has_string is 18
var has_string=$('*:contains("horsey rodeo")').length
//has_string if 0. So, you could an `if` on this and it would work as expected.