The Math.log1p() function returns the natural logarithm (base e) of 1 + a number, that is
Math.log1p()
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Syntax
Math.log1p(x)
Parameters
-
x -
A number.
Return value
Description
For very small values of x, adding 1 can reduce or eliminate precision. The double floats used in JS give you about 15 digits of precision. 1 + 1e-15 = 1.000000000000001, but 1 + 1e-16 = 1.000000000000000 and therefore exactly 1.0 in that arithmetic, because digits past 15 are rounded off.
When you calculate log(1 + x), you should get an answer very close to x, if x is small (that's why these are called 'natural' logarithms). If you calculate Math.log(1 + 1.1111111111e-15) you should get an answer close to 1.1111111111e-15. Instead, you will end up taking the logarithm of 1.00000000000000111022 (the roundoff is in binary so sometimes it gets ugly), so you get the answer 1.11022...e-15, with only 3 correct digits. If, instead, you calculate Math.log1p(1.1111111111e-15) you will get a much more accurate answer 1.1111111110999995e-15 with 15 correct digits of precision (actually 16 in this case).
If the value of x is less than -1, the return value is always NaN.
Because log1p() is a static method of Math, you always use it as Math.log1p(), rather than as a method of a Math object you created (Math is not a constructor).
Examples
Using Math.log1p()
Math.log1p(1); // 0.6931471805599453 Math.log1p(0); // 0 Math.log1p(-1); // -Infinity Math.log1p(-2); // NaN
Specifications
| Specification |
|---|
| ECMAScript Language Specification # sec-math.log1p |
Browser compatibility
| Desktop | Mobile | Server | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | Deno | Node.js | |
log1p |
38
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12
|
25
|
No
|
25
|
8
|
38
|
38
|
25
|
25
|
8
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3.0
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1.0
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0.12.0
|
See also
© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/log1p
