You have two possibilites:
(new RegExp('word')).test(str) // or /word/.test(str)
str.indexOf('word') !== -1
Regular expressions seem to be faster (at least in Chrome 10).
It cannot be said with certainty which method is faster. The differences between the browsers is enormous. While in Chrome 10
indexOf seems to be faster, in Safari 5,
indexOf is clearly slower than any other method.
You have to see and try for your self. It depends on your needs. For example a case-insensitive search is way faster with regular expressions.