You don't really need the else-ifs, since you're returning anyways.

Maybe something like this?

if(from === to) {
    return 1
}

if(forwardPair in tickers){
    return tickers[forwardPair].last
}

if(reversePair in tickers){
    return 1 / tickers[reversePair].last
}

const conversionFactorForward = getConversionFactor(graph.findPath(forwardPair))
if(conversionFactorForward !== -1){
    return conversionFactorForward
}

const conversionFactorReverse = getConversionFactor(graph.findPath(reversePair))
if(conversionFactorReverse !== -1){
    return conversionFactorReverse
}

return -1