function calcPrice (price, tax, discount) {
  tax = tax || 0.05
  discount = discount || 0

  return // math
}

calcPrice(10, 0, .1)

The || operator checks for falsy values and 0 is a falsy value. To fix this, instead of checking for a falsy value, you can check for undefined or just use ES6’s Default Parameters.

// es5
function calcPrice (price, tax, discount) {
  tax = typeof tax === 'undefined'
    ? 0.05
    : tax
  discount = typeof discount === 'undefined'
    ? 0
    : discount

  return // math
}

or using ES6’s default parameters

function calcPrice (price, tax = 0.05, discount = 0) {
  return // math
}