How Can I Determine if a String Represents a Valid Number in JavaScript?

How can I check if a string is a valid number?

In the programming world, being able to identify whether a string represents a valid number is essential. There are multiple approaches to tackle this task, each with its own nuances and applicability.

IsNumeric() Equivalent:

You mentioned the IsNumeric() function from VB6. While there's no direct equivalent in JavaScript, consider the following implementation:

function isNumeric(str) {
  if (typeof str != "string") return false;
  return !isNaN(str) && !isNaN(parseFloat(str));
}

Checking for Non-Numbers:

This method is straightforward and works for any variable, including strings:

isNaN(num) // returns true if variable does NOT contain a number

Converting to a Number:

To convert a numeric string to a number, ensure it only contains numeric characters:

+num // returns the numeric value of the string, or NaN otherwise

Loose Conversion (parseInt):

This method is helpful for extracting numbers from strings like '12px':

parseInt(num) // extracts a numeric value from the string's start

Floats:

Note that parseInt() converts floats to integers, discarding the decimal portion:

+12.345          // 12.345
parseInt(12.345) // 12

Empty Strings:

Empty strings have unexpected behaviors:

+' '             // 0
isNaN('')        // false
parseInt('')     // NaN

Choose the approach that best suits your needs, considering factors like string format and the desired output.

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