// Arrow function
reduce((previousValue, currentValue) => { ... } )
reduce((previousValue, currentValue, currentIndex) => { ... } )
reduce((previousValue, currentValue, currentIndex, array) => { ... } )
reduce((previousValue, currentValue, currentIndex, array) => { ... }, initialValue)

// Callback function
reduce(callbackFn)
reduce(callbackFn, initialValue)

// Inline callback function
reduce(function(previousValue, currentValue) { ... })
reduce(function(previousValue, currentValue, currentIndex) { ... })
reduce(function(previousValue, currentValue, currentIndex, array) { ... })
reduce(function(previousValue, currentValue, currentIndex, array) { ... }, initialValue)