從目標體脂推算理想體重與需減去的脂肪，用法為 weight( 100, 0.3, 0.2, 400 )，代表目前體重 100kg，體脂 30%，目標體脂 20%，每日運動可燒掉 400大卡。

weight.js

function weight(
	currentWeight = 82,
	currentRatio = 0.23,
	targetRatio = 0.2,
	dailyBurn = 400
) {
	const currentFat = currentWeight * currentRatio
	const currentMuscle = currentWeight - currentFat
	const targetWeight = currentMuscle / (1 - targetRatio)
	const loseFat = currentWeight - targetWeight // kg
	const calories = 7700 * loseFat // 大卡
	const estimatedDays = Math.round(calories / dailyBurn)

	return {
		currentFat,
		currentMuscle,
		targetWeight: Math.round(targetWeight),
		loseFat: Math.round( loseFat ),
		estimatedDays: `${Math.floor(estimatedDays / 30)} month ${estimatedDays % 30} days`,
	}
}

weight()
// { currentFat: 18.86, currentMuscle: 63.14, targetWeight: 78.925, loseFat: 3.075000000000003, estimatedDays: "1m 29d" }