JavaScript log1p

Returns log(1 + number), computed in a way that accurate even when the value of number is close to zero

function log1p (x) {
    // Returns log(1 + number), computed in a way that accurate even when the value of number is close to zero  
    // 
    // version: 1109.2015
    // discuss at: http://phpjs.org/functions/log1p    // +   original by: Brett Zamir (http://brett-zamir.me)
    // %          note 1: Precision 'n' can be adjusted as desired
    // *     example 1: log1p(1e-15);
    // *     returns 1: 9.999999999999995e-16
    var ret = 0,        n = 50; // degree of precision
    if (x <= -1) {
        return '-INF'; // JavaScript style would be to return Number.NEGATIVE_INFINITY
    }
    if (x < 0 || x > 1) {        return Math.log(1 + x);
    }
    for (var i = 1; i < n; i++) {
        if ((i % 2) === 0) {
            ret -= Math.pow(x, i) / i;        } else {
            ret += Math.pow(x, i) / i;
        }
    }
    return ret;}

external links: original PHP docs | raw js source

Examples

Running

1	log1p(1e-15);

Should return

1	9.999999999999995e-16

Dependencies

No dependencies, you can use this function standalone.

Open syntax issues

php.js uses JsLint to help us keep our code consistent and prevent some common bugs.

Eventually we want all code to pass or at least take into consideration most fixes suggested by JsLint, following this JsLint configuration we’ve decided on.

Authors

Thanks to the following developers, you get to have log1p goodness in JavaScript.

Brett Zamir ( original )

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