Handlebars currency helper and test

gistfile1.js

Handlebars.registerHelper('currency', function(amount, options) {
  if (typeof(amount) === 'string') { amount = options.contexts[0].get(amount); }
  var rounded = Math.round(amount * 100);
  var dec = rounded % 100;
  var whole = rounded / 100 - dec / 100;
  var decStr = '' + dec;
  return '$' + whole + '.' + decStr + ( decStr.length < 2 ? '0' : '');
});

test("USD", function () {

  //Fails
  var t = Handlebars.compile("{{currency 0.1}}");
  equal(t(), "$0.10", "Passed in cents");

  //Passes
  var t = Handlebars.compile("{{currency amount}}");
  equal(t({amount: 0.15}), "$0.15", "Passed in key");
});

Here It's better to replace #5 with var whole = Math.round(rounded / 100 - dec / 100);
because I've been able to get the following result: "0.9999999999999999.63"
It's when rounded is 163 and dec is 63
Then JS does crazy stuff with math and the whole will contain 0.9999999999999999
So I think adding Math.round would be better 😀

This function misinterprets cents when passing in an amount with 01 - 09 cents (ex 10.05 will output as "$10.50"). Try:

Handlebars.registerHelper('currency', function(amount, options) {
    var numericAmount = typeof amount === 'string' ? options.contexts[0].get(amount) : amount;
    var pennies = Math.round(numericAmount * 100);
    var cents = pennies % 100;
    var dollars = Math.round(pennies / 100 - cents / 100);
    var centsStr = '' + cents;
    var result = '$' + dollars + '.' + ( centsStr.length < 2 ? '0'+cents : cents);

    return '$' + dollars + '.' + ( centsStr.length < 2 ? '0' + cents : cents);
});

Once you have the amount you could just use built-in javascript methods to format the currency. Something like:

var num = new Number(amount);
var formattedNumber = num.toLocaleString('en-US', {style: 'currency', currency: 'USD'});

// Example
// amount: 2000.345
// output: $2,000.35 

@lazamar JSHint complains about using the Number constructor (since it's type is object instead of number). toLocaleString is available on primitives though: var x = 100; x.toLocaleString(...);

However it is not supported on all browsers, most notably Safari on iOS; it returns the original value.