negateValue.js

export default function negateValue(value) {
  value = `${value}`

  if (value === '0') {
    return '0'
  }

  // Flip sign of numbers
  if (/^[+-]?(\d+|\d*\.\d+)(e[+-]?\d+)?(%|\w+)?$/.test(value)) {
    return value.replace(/^[+-]?/, (sign) => (sign === '-' ? '' : '-'))
  }

  // What functions we support negating numeric values for
  // var() isn't inherently a numeric function but we support it anyway
  // The trigonometric functions are omitted because you'll need to use calc(…) with them _anyway_
  // to produce generally useful results and that will be covered already
  let numericFunctions = ['var', 'calc', 'min', 'max', 'clamp']

  for (const fn of numericFunctions) {
    if (value.includes(`${fn}(`)) {
      return `calc(${value} * -1)`
    }
  }
}