Checking and converting some value to number using conditional operator

Question

Given a string stored in a variable 'givenValue'. If it's all numbers, convert the string to number

(e.g. '11' to 11, 'a1' to 'a1')

and assign it to a variable 'value':

const value = givenValue - 0 === NaN ? givenValue : givenValue - 0;

But the output is not what I expected:

const givenValue = 'a1';
console.log(value); // NaN

const givenValue = '1';
console.log(value); // 1

Seems like the value of 'givenValue' is reassigned at the time of the 'if' condition being checked, or the condition check is not working.

Tân · Accepted Answer · 2019-12-25 02:41:00Z

3

You can use isNaN function to check something is NaN or not:

const givenValue = 'a1';
console.log(isNaN(givenValue) ? givenValue : +givenValue);

Also, if you want to check something is numeric or not before casting to a number, you can use isNaN function with isFinite function:

const givenValue = 'a1';

const value = !isNaN(parseFloat(givenValue)) && isFinite(givenValue) 
              ? givenValue : +givenValue;

console.log(value);

edited Dec 25, 2019 at 2:41

answered Dec 25, 2019 at 2:35

Tân

1

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Comments

CertainPerformance · Accepted Answer · 2019-12-25 02:37:02Z

3

Strings that aren't fully numbers but have numeric characters may well return a number other than NaN in many circumstances - see that link for a full description of how the algorithm works. Suffice to say, it's slightly complicated, and isn't what you're looking for. (eg, you'd want '123e456' to fail, but that'd actually give you Infinity instead. Whitespace will be permitted too.) (Also, a === NaN check will always return false, because NaN isn't equal to anything)

Instead, use a regular expression to check that the string contains only digits:

const value = /^\d+$/.test(givenValue) ? Number(givenValue) : givenValue;

If you want to include possible decimal amounts too, then add an optional group of . followed by digits:

const value = /^\d+(?:\.\d+)?$/.test(givenValue) ? Number(givenValue) : givenValue;
//                 ^^^^^^^^^^

edited Dec 25, 2019 at 2:37

answered Dec 25, 2019 at 2:31

CertainPerformance

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4 Comments

Tân Over a year ago

Can you please explain me the ?: part in your pattern?

CertainPerformance Over a year ago

That's an optional group of . followed by digits. Eg, it'll match the .123 part of 1.123 - it matches a . followed by \d+, digits

Tân Over a year ago

Because I see that you have ? follows by the group (\.\d+), so I think we can remove it ?:. The last ? mean: everything inside the group can be zero or one time, right?

CertainPerformance Over a year ago

The ?: at the beginning of the group makes it non-capturing. Capturing groups (denoted with just plain parentheses) should be used only when you need to capture the value - otherwise, better to use a non-capturing group, with (?:, to indicate to both the reader and the engine that the matched subpattern doesn't need to be saved or considered later.

Joseph Sible-Reinstate Monica · Accepted Answer · 2019-12-25 02:33:38Z

0

NaN doesn't equal itself. Try like this: givenValue - 0 !== givenValue - 0 ? givenValue : givenValue - 0.

answered Dec 25, 2019 at 2:33

Joseph Sible-Reinstate Monica

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Comments

Long Nguyen Duc · Accepted Answer · 2019-12-25 02:55:02Z

0

In Javascript, NaN === NaN is alway false.

So you should use isNaN(givenValue - 0) rather than givenValue - 0 === NaN

console.log(NaN === NaN) // false
console.log(NaN == NaN) // false
console.log(isNaN(NaN)) // true

const fixedFunc = (givenValue) => isNaN(givenValue - 0) ? givenValue : givenValue - 0;

console.log(JSON.stringify({
  "fixedFunc('a1')": fixedFunc('a1'),
  "fixedFunc('1')": fixedFunc('1')
}))

answered Dec 25, 2019 at 2:55

Long Nguyen Duc

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Comments

Yousername · Accepted Answer · 2019-12-25 03:54:12Z

-1

ONLY WORKS WITH NONZERO NUMBERS

A simpler method would be to use const value = givenValue - 0 || givenValue;

var givenValue = '1';
var value = givenValue - 0 || givenValue;
console.log(value);

givenValue = 'a1';
value = givenValue - 0 || givenValue;
console.log(value);

edited Dec 25, 2019 at 3:54

answered Dec 25, 2019 at 2:36

Yousername

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