30秒学会 JavaScript 片段 – How can I specify the base for a logarithm in JavaScript?

JavaScript’s Math.log(), Math.log2() and Math.log10() are useful for calculating logarithms in base e, 2 and 10 respectively. But what if you want to calculate the logarithm of a number in a different base? Or check if a number is a power of a specific base? That’s nearly as easy by using some math.

Calculate the logarithm of a number in a specific base

The base change formula allows you to calculate the logarithm of a number in a specific base by dividing the logarithm of the number by the logarithm of the base. This means that for a given base b and a number n you can calculate the logarithm of n in base b by using the formula Math.log(n) / Math.log(b).

代码实现

const logBase = (b, n) => Math.log(n) / Math.log(base);

logBase(5, 25); // 2
logBase(5, 625); // 4

Using partial application, you can create a function that calculates the logarithm of a number in a specific base by only passing the base as an argument. This returns a new function that takes the number as an argument and calculates the logarithm of the number in the specified base.

使用样例

const logBase = b => n => Math.log(n) / Math.log(base);

const logBase5 = logBase(5);

logBase5(25); // 2
logBase5(625); // 4

Check if a number is a power of a specific base

In order to check if a number is a power of a specific base, you need to calculate its logarithm in that base first. Then, you can use the modulo operator (%) to check if the result is an integer. If it is, the number is a power of the specified base, otherwise it isn’t.

const isPowerOf10 = n => Math.log10(n) % 1 === 0;
const isPowerOf2 = n => Math.log2(n) % 1 === 0;
const isPowerOf = b => n => Math.log(n) / Math.log(b) % 1 === 0;

isPowerOf10(1); // true
isPowerOf10(10); // true
isPowerOf10(20); // false

isPowerOf2(1); // true
isPowerOf2(2); // true
isPowerOf2(3); // false

const isPowerOf5 = isPowerOf(5);

isPowerOf5(5, 25); // true
isPowerOf5(5, 625); // true
isPowerOf5(5, 20); // false

[!TIP]

Favor the built-in logarithmic functions, whenever available, as they are likely to be more performant than a custom implementation.

翻译自：https://www.30secondsofcode.org/js/s/specify-base-for-math-log