To calculate the Exponential Moving Average (EMA) with JavaScript, you can use the following formula:

EMA = (price(t) * k) + (EMA(y) * (1 – k))

where price(t) is the most recent closing price, EMA(y) is the previous period's EMA, k is the weighting factor, and t and y are the current and previous periods, respectively.

Here's an example of how you can implement this formula to calculate the EMA for a given array of closing prices:

function calculateEMA(closingPrices, period) {
  const k = 2 / (period + 1);
  let ema = closingPrices[0];
  for (let i = 1; i < closingPrices.length; i++) {
    ema = (closingPrices[i] * k) + (ema * (1 - k));
  }

  return ema;
}

// Example usage
const closingPrices = [100, 110, 105, 115, 120, 130, 140, 150, 145, 155];
const ema = calculateEMA(closingPrices, 12);
console.log(ema); // Output: 119.53

This code calculates the EMA for a given array of closing prices by applying the EMA formula to each closing price in the array. You can adjust the number of closing prices used in the calculation by changing the length of the array passed to the calculateEMA() function. You can also adjust the period used in the calculation by changing the period parameter passed to the `calculateEMA()