为什么这个 if 语句以非布尔值成功？

Question

var m = 5;
if (m) {
    document.write(m);
}

In my view, m is not a bool type, therefore, this if statement will not be executed. However, I do see the value of m printed on my screen. Why?

Answer 1

You should read about truthy values ( MDN ).

In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy (ie, except for false , 0 , 0n , "" , null , undefined , and NaN ).

Any number other than 0 is truthy, so if(5) is the same as if(true) .