`Math.atanh()` 函数返回一个数值反双曲正切值, 即：

∀x∊(-1,1),Math.atanh(x)=arctanh(x)= the unique y such that tanh(y)=x\forall x \in \left( -1, 1 \right), \mathtt{\operatorname{Math.atanh}(x)} = \operatorname{arctanh}(x) = \text{ the unique } \; y \; \text{such that} \; \tanh(y) = x

## 语法

    Math.atanh(x)

### 参数

`x`
  一个数值

## 描述

由于 `atanh()` 是 `Math 的静态方法`,所以应该像这样使用：`Math.atanh()`，而不是作为你创建的 `Math` 实例的方法（`Math 不是一个构造函数`）。

## 示例

### `使用 Math.atanh()`

    Math.atanh(-2);  // NaN
    Math.atanh(-1);  // -Infinity
    Math.atanh(0);   // 0
    Math.atanh(0.5); // 0.5493061443340548
    Math.atanh(1);   // Infinity
    Math.atanh(2);   // NaN

对于大于1或是小于－1的值，函数返回 `NaN` 。

## Polyfill

For \left|x\right| < 1, we have \operatorname {artanh} (x) = \frac{1}{2}\ln \left( \frac{1 + x}{1 - x} \right) so this can be emulated by the following function:

    Math.atanh = Math.atanh || function(x) {
      return Math.log((1+x)/(1-x)) / 2;
    };

## 规范

Specification | Status | Comment
--- | --- | ---
ECMAScript 2015 (6th Edition, ECMA-262) Math.atanh | Standard | Initial definition.
ECMAScript Latest Draft (ECMA-262) Math.atanh | Draft |

## 浏览器兼容性

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

- Desktop
- Mobile

Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari
--- | --- | --- | --- | --- | ---
Basic support | 38 | 25 (25) | 未实现 | 25 | 7.1

Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile
--- | --- | --- | --- | --- | --- | ---
Basic support | 未实现 | 未实现 | 25.0 (25) | 未实现 | 未实现 | 8

## 相关链接

- `Math.acosh()`
- `Math.asinh()`
- `Math.cosh()`
- `Math.sinh()`
- `Math.tanh()`