Javascript and its !! (double exclamation mark) opertaor

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本文通过一个具体的代码示例介绍了JavaScript中!!运算符的作用及其使用场景。!!运算符可以将任何表达式转换为其布尔等价形式，例如将函数转换为布尔值true。

Let's first see a code example that has !! operator turned on

var store = { 
  id : 1 ,
  cache : {},
  add : function(fn) {
    if (!fn.id) { 
      fn.id = store.id++;
      return !!(store.cache[fn.id] = fn);
    }
  }
};

function ninja() {}

assert (store.add(ninja), "Function was safely added.");
assert(store.add(ninja), "But it was only added once.");

you may wonder what is the operator of !!...

the explanation of the !! operator is such, "!! construct is a simple way of turning any JavaScript expression into its boolean equivalent, for example !!"hello" === true and !!0 === false. "

In the above example, we converted a function ( which will always be true ) into its boolean equivalent: true.