Number between double quotes

Question

According to this code

function sayHi(myAge) {
    "use strict";
    if (isNaN(myAge)) {
        return "Ture";
    } else {
        return "False";
    }
}
sayHi("12");

isNan() return false, Why? "12" is not a number.

Because When I do this

var myAge = "12";
alert(myAge === 12);

it will return false, because "12" is a string but 12 a number.

What does `alert(myAge == 12)` return? "12" is converted into a number. When you check with === it is strict equality. — Dominique Lorre, Sep 14 '16 at 16:26
Are you trying to validate numbers? Maybe you're looking for something like: http://stackoverflow.com/a/9716488/63011 — Paolo Moretti, Sep 14 '16 at 16:28
@DominiqueLorre return `true` , That's what I am looking for, I am just want to figure out why isNan() return `false` for `"12"` — MMJ, Sep 14 '16 at 16:34
NaN is a special value, which means "undefined" or "not possible to display". Check [this](https://en.wikipedia.org/wiki/NaN) — Dominique Lorre, Sep 14 '16 at 16:40

score 2 · Answer 1 · answered Sep 14 '16 at 16:24

2

From the spec:

Returns true if the argument coerces to NaN, and otherwise returns false.

Compare to ===:

If Type(x) is different from Type(y), return false.

answered Sep 14 '16 at 16:24

Quentin

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Alexey Katayev · Accepted Answer · 2016-09-14T16:28:53.710

2

Because NaN is a special value in JS, not a type. sayHi(NaN) will return true.

If you want to check if the value is the Number type, you should do

if (typeof myAge === "number")

And if you want to be sure, that it's not NaN as well, then

if (typeof myAge === "number" && !isNaN(myAge))

edited Sep 14 '16 at 16:28

answered Sep 14 '16 at 16:26

Alexey Katayev

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2
10

`NaN` is technically a `typeof` `"number"` – Sterling Archer Sep 14 '16 at 16:27
@SterlingArcher damn, sorry – Alexey Katayev Sep 14 '16 at 16:28
So `"12"` isn't NaN but still a String type, Got it, Thanks. – MMJ Sep 14 '16 at 16:49

score 1 · Answer 3 · answered Sep 14 '16 at 16:26

The isNaN() function determines whether a value is NaN or not. Note: coercion inside the isNaN function has interesting rules; you may alternatively want to use Number.isNaN(), as defined in ECMAScript 6, or you can use typeof to determine if the value is Not-A-Number.

Reference : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

Number between double quotes

3 Answers3