locutus/esm/php/math/log1p.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

export function log1p(x) {
    //      discuss at: https://locutus.io/php/log1p/
    // parity verified: PHP 8.3
    //     original by: Brett Zamir (https://brett-zamir.me)
    //     improved by: Robert Eisele (https://www.xarg.org/)
    //          note 1: Precision 'n' can be adjusted as desired
    //       example 1: log1p(1e-15)
    //       returns 1: 9.999999999999995e-16
    let ret = 0;
    // degree of precision
    const n = 50;
    if (x <= -1) {
        // JavaScript style would be to return Number.NEGATIVE_INFINITY
        return '-INF';
    }
    if (x < 0 || x > 1) {
        return Math.log(1 + x);
    }
    for (let i = 1; i < n; i++) {
        ret += Math.pow(-x, i) / i;
    }
    return -ret;
}