javascript - 有人能解释这个“双重否定”的伎俩吗？ [重复](Can someone explain this 'double negative' trick? [duplicate])

Question

This question already has an answer here:

(这个问题在这里已有答案：)

• What is the !!

  (是什么 ！！)

  (not not) operator in JavaScript?

  (（不是）JavaScript中的运算符？)

  35 answers

  (35个答案)

I am by no means an expert at Javascript, but I have been reading Mark Pilgrim's "Dive into HTML5" webpage and he mentioned something that I would like a better understanding of.

(我绝不是Javascript的专家，但我一直在阅读Mark Pilgrim的“潜入HTML5”网页，他提到了一些我想要更好理解的内容。)

He states:

(他说：)

Finally, you use the double-negative trick to force the result to a Boolean value (true or false).

(最后，使用双负技巧将结果强制为布尔值（true或false）。)

function supports_canvas() {
  return !!document.createElement('canvas').getContext;
}

If anyone can explain this a little better I would appreciate it!

(如果有人能够更好地解释这一点我会很感激！)

  ask by ProfessionalAmateur translate from so

Answer 1

A logical NOT operator !

(逻辑NOT运算符!)

converts a value to a boolean that is the opposite of its logical value.

(将值转换为与其逻辑值相反的布尔值。)

The second !

(第二个!)

converts the previous boolean result back to the boolean representation of its original logical value.

(将前一个布尔结果转换回其原始逻辑值的布尔表示。)

From these docs for the Logical NOT operator:

(从 Logical NOT运算符的这些文档中：)

Returns false if its single operand can be converted to true;

(如果单个操作数可以转换为true，则返回false;)

otherwise, returns true.

(否则，返回true。)

So if getContext gives you a "falsey" value, the !!

(所以如果getContext给你一个“假”值，那就!!)

will make it return the boolean value false .

(将使它返回布尔值false 。)

Otherwise it will return true .

(否则它将返回true 。)

The "falsey" values are:

(“假”值是：)

• false
• NaN
• undefined
• null
• "" (empty string)

  ("" （空字符串）)

• 0