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Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.

Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.

Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.

Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.

Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(parseFloat(n)) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.

Only problem I had with @CMS's answer is the exclusion of NaN and Infinity, which are useful numbers for many situations. One way to check for NaN's is to check for numeric values that don't equal themselves, NaN != NaN! So there are really 3 tests you'd like to deal with ...

function isNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) || n != n;
}
function isFiniteNumber(n) {
  n = parseFloat(n);
  return !isNaN(n) && isFinite(n);
}    
function isComparableNumber(n) {
  n = parseFloat(n);
  return (n >=0 || n < 0);
}

isFiniteNumber('NaN')
false
isFiniteNumber('OxFF')
true
isNumber('NaN')
true
isNumber(1/0-1/0)
true
isComparableNumber('NaN')
false
isComparableNumber('Infinity')
true

My isComparableNumber is pretty close to another elegant answer, but handles hex and other string representations of numbers.