javascript - Validate that a string is a positive integer

Question

I would like the simplest fail-safe test to check that a string in JavaScript is a positive integer.

isNaN(str) returns true for all sorts of non-integer values and parseInt(str) is returning integers for float strings, like "2.5". And I don't want to have to use some jQuery plugin either.

Answer 1

Two answers for you:

• Based on parsing

• Regular expression

Note that in both cases, I've interpreted "positive integer" to include 0, even though 0 is not positive. I include notes if you want to disallow 0.

Based on Parsing

If you want it to be a normalized decimal integer string over a reasonable range of values, you can do this:

function isNormalInteger(str) {
    var n = Math.floor(Number(str));
    return n !== Infinity && String(n) === str && n >= 0;
}

or if you want to allow whitespace and leading zeros:

function isNormalInteger(str) {
    str = str.trim();
    if (!str) {
        return false;
    }
    str = str.replace(/^0+/, "") || "0";
    var n = Math.floor(Number(str));
    return n !== Infinity && String(n) === str && n >= 0;
}

Live testbed (without handling leading zeros or whitespace):

function isNormalInteger(str) {
    var n = Math.floor(Number(str));
    return n !== Infinity && String(n) === str && n >= 0;
}
function gid(id) {
    return document.getElementById(id);
}
function test(str, expect) {
    var result = isNormalInteger(str);
    console.log(
        str + ": " +
        (result ? "Yes" : "No") +
        (expect === undefined ? "" : !!expect === !!result ? " <= OK" : " <= ERROR ***")
    );
}
gid("btn").addEventListener(
    "click",
    function() {
        test(gid("text").value);
    },
    false
);
test("1", true);
test("1.23", false);
test("1234567890123", true);
test("1234567890123.1", false);
test("0123", false); // false because we don't handle leading 0s
test(" 123 ", false); // false because we don't handle whitespace

<label>
  String:
  <input id="text" type="text" value="">
<label>
<input id="btn" type="button" value="Check">