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The JavaScript FAQ
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Number vs. String

Question: Is there a way to test whether a particular variable holds a number or a string?

Answer: Yes. To test whether the variable holds a number or a string, use the typeof operator. If your variable holds a number, typeof(variable) will return "number". If it holds a string, typeof(variable) will return "string". The following are examples of typeof usage:

typeof(123)    // result: "number"
typeof("123")  // result: "string"

if (typeof k == "string") { alert('k is a string.') }
if (typeof k == "number") { alert('k is a number.') }
The typeof operator can also help you distinguish between other data types. Depending on the particular variable's value, the result of typeof can be one of the following:
    "undefined" Copyright © 1999-2006, Alexei Kourbatov

The JavaScript FAQ
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  Mirrored with kind permission of Alexei Kourbatov Design by Interspire