Question

I’m hoping there’s something in the same conceptual space as the old VB6 IsNumeric() function?

Answer 1

2nd October 2020: note that many bare-bones approaches are fraught with subtle bugs (eg. whitespace, implicit partial parsing, radix, coercion of arrays etc.) that many of the answers here fail to take into account. The following implementation might work for you, but note that it does not cater for number separators other than the decimal point ".":

function isNumeric(str) {   if (typeof str != "string") return false // we only process strings!     return !isNaN(str) && // use type coercion to parse the _entirety_ of the string (`parseFloat` alone does not do this)...          !isNaN(parseFloat(str)) // ...and ensure strings of whitespace fail } 

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To check if a variable (including a string) is a number, check if it is not a number:

This works regardless of whether the variable content is a string or number.

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

Examples

isNaN(123)         // false isNaN('123')       // false isNaN('1e10000')   // false (This translates to Infinity, which is a number) isNaN('foo')       // true isNaN('10px')      // true isNaN('')          // false isNaN(' ')         // false isNaN(false)       // false 

Of course, you can negate this if you need to. For example, to implement the IsNumeric example you gave:

function isNumeric(num){   return !isNaN(num) } 

To convert a string containing a number into a number:

Only works if the string only contains numeric characters, else it returns NaN.

+num               // returns the numeric value of the string, or NaN                     // if the string isn't purely numeric characters 

Examples

+'12'              // 12 +'12.'             // 12 +'12..'            // NaN +'.12'             // 0.12 +'..12'            // NaN +'foo'             // NaN +'12px'            // NaN 

To convert a string loosely to a number

Useful for converting ’12px’ to 12, for example:

parseInt(num)      // extracts a numeric value from the                     // start of the string, or NaN. 

Examples

parseInt('12')     // 12 parseInt('aaa')    // NaN parseInt('12px')   // 12 parseInt('foo2')   // NaN      These last three may parseInt('12a5')   // 12       be different from what parseInt('0x10')   // 16       you expected to see. 

Floats

Bear in mind that, unlike +num, parseInt (as the name suggests) will convert a float into an integer by chopping off everything following the decimal point (if you want to use parseInt() because of this behaviour, you’re probably better off using another method instead):

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

Empty strings

Empty strings may be a little counter-intuitive. +num converts empty strings or strings with spaces to zero, and isNaN() assumes the same:

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

But parseInt() does not agree:

parseInt('')       // NaN parseInt('   ')    // NaN