Best way to check for IE less than 9 in JavaScript without library

Question

What would be your fastest, shortest (best) way to detect browser which is IE and version less than 9 in JavaScript, without using jQuery or any add-on libraries?

Answer 1

if (+(/MSIE\s(\d+)/.exec(navigator.userAgent)||0)[1] < 9) {
    // IE8 or less
}

• extract IE version with: /MSIE\s(\d+)/.exec(navigator.userAgent)
• if it's non-IE browser this will return null so in that case ||0 will switch that null to 0
• [1] will get major version of IE or undefined if it was not an IE browser
• leading + will convert it into a number, undefined will be converted to NaN
• comparing NaN with a number will always return false