javascript 两个感叹号

var foo1,foo2 = 0,foo3 = "",foo4 = null,foo5 = 1,foo6 = "0",foo7 = "1",foo8 = [],foo9 = {};document.write("var: " + !!foo1 + "<br>"); //falsedocument.write("0: " + !!foo2 + "<br>"); //falsedocument.write("\"\": " + !!foo3 + "<br>"); //fasledocument.write("null: " + !!foo4 + "<br>"); //falsedocument.write("1: " + !!foo5 + "<br>"); //truedocument.write("\"0\": " + !!foo6 + "<br>"); //truedocument.write("\"1\": " + !!foo7 + "<br>"); //truedocument.write("[]: " + !!foo8 + "<br>"); //truedocument.write("{}: " + !!foo9 + "<br>"); //true