Add .00 (toFixed) only if number has less than two decimal places

The below code provides one way to do what you want. There are others.

function addZeroes(num) {
    // Cast as number
    var num = Number(num);
    // If not a number, return 0
    if (isNaN(num)) {
        return 0;
    }
    // If there is no decimal, or the decimal is less than 2 digits, toFixed
    if (String(num).split(".").length < 2 || String(num).split(".")[1].length<=2 ){
        num = num.toFixed(2);
    }
    // Return the number
    return num;
}

console.log(addZeroes(5)); // Alerts 5.00
console.log(addZeroes(5.1)); // Alerts 5.10
console.log(addZeroes(5.11)); // Alerts 5.11
console.log(addZeroes(5.111)); // Alerts 5.111

http://jsfiddle.net/nzK4n/

Here you go:

function addZeroes(num) {
// Convert input string to a number and store as a variable.
    var value = Number(num);      
// Split the input string into two arrays containing integers/decimals
    var res = num.split(".");     
// If there is no decimal point or only one decimal place found.
    if(res.length == 1 || res[1].length < 3) { 
// Set the number to two decimal places
        value = value.toFixed(2);
    }
// Return updated or original number.
return value;
}

// If you require the number as a string simply cast back as so
var num = String(value);

See fiddle for demonstration.

edit: Since I first answered this, javascript and I have progressed, here is an improved solution using ES6, but following the same idea:

function addZeroes(num) {
  const dec = num.split('.')[1]
  const len = dec && dec.length > 2 ? dec.length : 2
  return Number(num).toFixed(len)
}

Updated fiddle

edit 2: Or if you are using optional chaining you can do it in one line like so:

const addZeroes = num => Number(num).toFixed(Math.max(num.split('.')[1]?.length, 2) || 2)

Updateder fiddle

Maybe use .toLocaleString():

var num = 5.1;    
var numWithZeroes = num.toLocaleString("en",{useGrouping: false,minimumFractionDigits: 2});
console.log(numWithZeroes);

As a function/demo:

function addZeroes(num) {
   return num.toLocaleString("en", {useGrouping: false, minimumFractionDigits: 2})
}

console.log('before   after       correct');
console.log('5      ->', addZeroes(5) , '  --> 5.00');
console.log('5.1    ->', addZeroes(5.1) , '  --> 5.10');
console.log('5.11   ->', addZeroes(5.11) , '  --> 5.11 (no change)');
console.log('5.111  ->', addZeroes(5.111) , ' --> 5.111 (no change)');
console.log('5.1111 ->', addZeroes(5.1111) , '--> 5.1111 (no change)');
console.log('-5     ->', addZeroes(-5) , ' --> -5.00');

And if you must use .toFixed(), here's a one-liner:

var num = 5.1;    
var numWithZeroes = num.toFixed(Math.max(((num+'').split(".")[1]||"").length, 2));
console.log(numWithZeroes);

Or, again, as a function/demo:

function addZeroes(num) {
   return num.toFixed(Math.max(((num+'').split(".")[1]||"").length, 2));
}

console.log('before   after       correct');
console.log('5      ->', addZeroes(5) , '  --> 5.00');
console.log('5.1    ->', addZeroes(5.1) , '  --> 5.10');
console.log('5.11   ->', addZeroes(5.11) , '  --> 5.11 (no change)');
console.log('5.111  ->', addZeroes(5.111) , ' --> 5.111 (no change)');
console.log('5.1111 ->', addZeroes(5.1111) , '--> 5.1111 (no change)');
console.log('-5     ->', addZeroes(-5) , ' --> -5.00');