_（下划线）变量的赋值

【Assignment of the _ (underscore) variable】

默认评估器会将最近评估的表达式结果默认赋值给特殊变量 _（下划线）。显式将 _ 设置为某个值将会禁用此行为。

【The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable _ (underscore). Explicitly setting _ to a value will disable this behavior.】

> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4 拷贝

同样地，_error 将指代最后一次出现的错误（如果有的话）。显式地将 _error 设置为某个值将会禁用此行为。

【Similarly, _error will refer to the last seen error, if there was any. Explicitly setting _error to a value will disable this behavior.】

> throw new Error('foo');
Uncaught Error: foo
> _error.message
'foo' 拷贝