Question

我对如何评估truthy或falsy值的角度有点困惑。显然，当我直接以有棱角的方式做以下事情时，事情就会起作用。

if (0) { console.log('not printed'); }
else { console.log('printed'); }

if (1) { console.log('printed'); }
else { console.log('not printed'); }

但是，如果我从模板中将值传递给我的控制器......那么事情就不起作用了。为什么呢？

function (someValue) {

    if (someValue) { console.log('always printed  whether someValue is 0 or 1'); }
    else { console.log('not printed'); }

...

Answer 1

比较时，js中存在松散相等和严格相等。如果该值不是Boolean的类型，则会转换为true或false

if (someValue === '0' || someValue === '1') { 
  console.log('always printed  whether someValue is 0 or 1'); 
}
else { 
  console.log('not printed'); 
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

Implicit data type conversion in JavaScript when comparing integer with string using ==

Answer 2

请勿在{{1}}语句中使用0和1。因为在javascript中使用try if和'0'作为字符串或设置其他一些标志来克服这个问题是很困惑的。

Answer 3

通常我使用全局函数来推导truthy值。

function truthy(val){
  switch(typeof val){
    case "number":
      // since the val is already a number, only zero and NaN should be counted as falsy.
      return val !== 0 && !Number.isNaN(Number(val));
    case "string":
      // val is string, so need to check if its empty or "0". Otherwise it should be counted as truthy
      return val !== "" && val !== "0";
    case "boolean":
      // val is already boolean so only boolean check would suffice
      return val === true;
    case "symbol":
      //    symbol should be truthy
      return true;
    case "object":
      // object can be deceiving, so lets check for NULL and empty object

      if( val === null ){
        return false;
      }
      else{
        for( var i in val ){
          if( val.hasOwnProperty(i) ){
            return true;
          }
        }
      }

      return false;

    case "undefiend":
      return false;
    case "function":
      return true;
  }
}
function falsy(val){
  return !truthy(val);
}

console.log(truthy(1));     // should be true
console.log(truthy(0));     // should be false
console.log(truthy(NaN));   // should be false
console.log(truthy(""));    // should be false
console.log(truthy("0"));   // should be false
console.log(truthy("1"));   // should be true
console.log(truthy("meh")); // should be true
console.log(truthy({}));    // should be false
console.log(truthy(null));  // should be false
console.log(truthy(false)); // should be false

http://jsbin.com/wakiyej/edit?js,console

为什么在Angular控制器中没有真正的值或虚假值？

3 个答案: