str.indexOf(substr) == 0
str.slice(0, 10) == substr
Which one of the above two is better? Is there any better way?
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related: http://stackoverflow.com/questions/646628/javascript-startswith – ajax333221 May 25 '12 at 22:52
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The performance of the first depends on the length of the string str as it requires scanning the entire string. If str is large it could take a long time to find whether or not the substring is contained within it.
If your input string could be large prefer the second.
Another option is:
str.lastIndexOf(substr, 0) == 0
This avoids scanning the entire string and avoids creating a temporary string.
If performance matters, I'd suggest measuring it for your specific data.
Mark Byers
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A (very) quick test suggests that the difference is probably negligible in Chrome, but that Firefox is substantially quicker with indexOf (for short strings, at least).
lonesomeday
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Safari has an even greater difference. Closer to 3x. +1 for the perf test. I was too lazy. ;o) – user113716 Jan 02 '11 at 15:49
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1I never knew about jsperf.com (thanks), but when I run it win7 + chrome slice wins :-/ – David Jan 02 '11 at 15:57
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Off the top of my head, I'd say to use:
str.indexOf(substr) == 0;
Simply because it doesn't need to create a new string from the first. Performance testing would be needed. Could vary by browser.
user113716
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Performance shouldn't be an issue. Creating a 10 character string will take less than negible time anyway. – Jan 02 '11 at 15:42
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@delnan: I agree, but the question was asked, and for me this version edges out the other simply because of the fact that the other creates a new String that just gets discarded after the comparison. – user113716 Jan 02 '11 at 15:45