与PHP函数等效的Javascript函数[重复]

I have the follwing code in PHP:

if(is_numeric($first_name[0]) === true){
   //Do something
}

How would I be able to do the same check using JavaScript? I also would like to get a PHP script to check if there is a number in $first_name at all, as I don't want the user to add a number in their first or last names please?

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Use regular expressions.

if (/-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/.test(yourInput)) {
   // Do something
}

Of course, this will only work on strings. For a different method, see a duplicate question:

function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}

To only check if the input contains a number (0-9) at all, regex works again:

if (/[0-9]+/.test(yourInput)) {
   // Do something
}

If the variable in question is $first_name

if( $first_name && !isNaN($first_name[0])) {

    // do something
}

in js to check if the number variable is a number:

isFinite(number) && !isNaN(parseFloat(number))

Something I found here:

function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}

If you want to check whether *$first_name* contains any digits, then:

if (/\d/.test($first_name)) {
    // $first_name contains at least one digit
}