Printing engineering units in typescrit

unit.ts

const posPrefix = ["", "k", "M", "G", "T", "P"]
const negPrefix = ["", "m", "μ", "n", "p", "a"]

/**
 * Convert a number to a metric string in engineering/SI notation. For example,
 * toMetric(1.2345678e8, "Wh", 4) returns "123.5MWh".
 * @param value The number to convert to a string.
 * @param unit A string representing the unit to use e.g. kW, C, V expected
 * @param The total number of digits to return
 */

function toMetric(value: number, unit: string, digits = 4): string {
	const sign = Math.sign(value) >= 0 ? "": "-"
	const log = Math.log10(Math.abs(value))
	const exp = Math.floor(log)
	const idx = ~~(exp / 3) // Dang but this is an ugly way to do div. We could use bigints if this becomes a problem
	const prefix = idx >= 0 ? posPrefix[idx]: negPrefix[-idx]
	const point = exp > 0 ? digits - exp % 3 - 1 : digits + Math.abs(exp) % 3 - 1
	const norm = Math.pow(10, log - exp + exp % 3)
	const str = norm.toFixed(point)
return sign + str + prefix + unit
}

console.log(toMetric(1.2345e-12, "C"))
console.log(toMetric(1.2345e-11, "C"))
console.log(toMetric(1.2345e-10, "C"))
console.log(toMetric(1.2345e-9, "C"))
console.log(toMetric(1.2345e-8, "C"))
console.log(toMetric(1.2345e-7, "C"))
console.log(toMetric(1.2345e-6, "C"))
console.log(toMetric(1.2345e-5, "C"))
console.log(toMetric(1.2345e-4, "C"))
console.log(toMetric(1.2345e-3, "C")) // 1.234mC
console.log(toMetric(1.2345e-2, "C")) // 12.34mC
console.log(toMetric(1.2345e-1, "V")) // 0.1234mV
console.log(toMetric(1.2345, "Wh"))
console.log(toMetric(1.2345e1, "Wh"))
console.log(toMetric(1.2345e2, "Wh"))
console.log(toMetric(1.2345e3, "Wh"))
console.log(toMetric(1.2345e4, "Wh"))
console.log(toMetric(1.2345e5, "Wh"))
console.log(toMetric(1.2345e6, "Wh"))
console.log(toMetric(1.2345e7, "Wh"))
console.log(toMetric(1.2345e8, "Wh"))
console.log(toMetric(1.2345e9, "Wh"))
console.log(toMetric(1.2345e10, "Wh"))
console.log(toMetric(1.2345e11, "Wh"))
console.log(toMetric(1.2345e12, "Wh"))