Number(1).toFixed(2);         // 1.00
Number(1.341).toFixed(2);     // 1.34
Number(1.345).toFixed(2);     // 1.34 NOTE: See andy's comment below.
Number(1.3450001).toFixed(2); // 1.35

document.getElementById('line1').innerHTML = Number(1).toFixed(2);
document.getElementById('line2').innerHTML = Number(1.341).toFixed(2);
document.getElementById('line3').innerHTML = Number(1.345).toFixed(2);
document.getElementById('line4').innerHTML = Number(1.3450001).toFixed(2);

<span id="line1"></span>
<br/>
<span id="line2"></span>
<br/>
<span id="line3"></span>
<br/>
<span id="line4"></span>

Number.prototype.toFixed

This solution is compatible with every single major browser:

  const profits = 2489.8237;

  profits.toFixed(3) // Returns 2489.824 (rounds up)
  profits.toFixed(2) // Returns 2489.82
  profits.toFixed(7) // Returns 2489.8237000 (pads the decimals)

All you need is to add the currency symbol (e.g. "$" + profits.toFixed(2)) and you will have your amount in dollars.

Custom function

If you require the use of , between each digit, you can use this function:

function formatMoney(number, decPlaces, decSep, thouSep) {
    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces,
    decSep = typeof decSep === "undefined" ? "." : decSep;
    thouSep = typeof thouSep === "undefined" ? "," : thouSep;
    var sign = number < 0 ? "-" : "";
    var i = String(parseInt(number = Math.abs(Number(number) || 0).toFixed(decPlaces)));
    var j = (j = i.length) > 3 ? j % 3 : 0;

    return sign +
        (j ? i.substr(0, j) + thouSep : "") +
        i.substr(j).replace(/(\decSep{3})(?=\decSep)/g, "$1" + thouSep) +
        (decPlaces ? decSep + Math.abs(number - i).toFixed(decPlaces).slice(2) : "");
}

document.getElementById("b").addEventListener("click", event => {
  document.getElementById("x").innerText = "Result was: " + formatMoney(document.getElementById("d").value);
});

<label>Insert your amount: <input id="d" type="text" placeholder="Cash amount" /></label>
<br />
<button id="b">Get Output</button>
<p id="x">(press button to get output)</p>

Use it like so:

(123456789.12345).formatMoney(2, ".", ",");

If you're always going to use '.' and ',', you can leave them off your method call, and the method will default them for you.

(123456789.12345).formatMoney(2);

If your culture has the two symbols flipped (i.e., Europeans) and you would like to use the defaults, just paste over the following two lines in the formatMoney method:

    d = d == undefined ? "," : d,
    t = t == undefined ? "." : t,

Custom function (ES6)

If you can use modern ECMAScript syntax (i.e., through Babel), you can use this simpler function instead:

function formatMoney(amount, decimalCount = 2, decimal = ".", thousands = ",") {
  try {
    decimalCount = Math.abs(decimalCount);
    decimalCount = isNaN(decimalCount) ? 2 : decimalCount;

    const negativeSign = amount < 0 ? "-" : "";

    let i = parseInt(amount = Math.abs(Number(amount) || 0).toFixed(decimalCount)).toString();
    let j = (i.length > 3) ? i.length % 3 : 0;

    return negativeSign +
      (j ? i.substr(0, j) + thousands : '') +
      i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + thousands) +
      (decimalCount ? decimal + Math.abs(amount - i).toFixed(decimalCount).slice(2) : "");
  } catch (e) {
    console.log(e)
  }
};

document.getElementById("b").addEventListener("click", event => {
  document.getElementById("x").innerText = "Result was: " + formatMoney(document.getElementById("d").value);
});

<label>Insert your amount: <input id="d" type="text" placeholder="Cash amount" /></label>
<br />
<button id="b">Get Output</button>
<p id="x">(press button to get output)</p>

You can limit the keys in the keydown event instead of keyup and allow specific keys to take effect, and then format the input on keyup event:

$("#testinput").on("keydown", function(e) {
  var keycode = (event.which) ? event.which : event.keyCode;
  if (e.shiftKey == true || e.ctrlKey == true) return false;
  if ([8, 110, 39, 37, 46].indexOf(keycode) >= 0 || // allow tab, dot, left and right arrows, delete keys
    (keycode == 190 && this.value.indexOf('.') === -1) || // allow dot if not exists in the value
    (keycode == 110 && this.value.indexOf('.') === -1) || // allow dot if not exists in the value
    (keycode >= 48 && keycode <= 57) || // allow numbers
    (keycode >= 96 && keycode <= 105)) { // allow numpad numbers
    // check for the decimals after dot and prevent any digits
    var parts = this.value.split('.');
    if (parts.length > 1 && // has decimals
      parts[1].length >= 2 && // should limit this
      (
        (keycode >= 48 && keycode <= 57) || (keycode >= 96 && keycode <= 105)
      ) // requested key is a digit
    ) {
      return false;
    } else {
      if (keycode == 110) {
        this.value += ".";
        return false;
      }
      return true;
    }
  } else {
    return false;
  }
}).on("keyup", function() {
  var parts = this.value.split('.');
  parts[0] = parts[0].replace(/,/g, '').replace(/^0+/g, '');
  if (parts[0] == "") parts[0] = "0";
  var calculated = parts[0].replace(/(\d)(?=(\d{3})+(?!\d))/g, "$1,");
  if (parts.length >= 2) calculated += "." + parts[1].substring(0, 2);
  this.value = calculated;
  if (this.value == "NaN" || this.value == "") this.value = 0;
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="text" id="testinput">