How do I negate the parameter for AngularJS ng-if

问题:

Quick Example:

There is a routed parameter (/Home/:isLoggedIn) that equates to true or false. (/Demo/#/Home/false) and a controller property

this.loggedIn = this.routeParams.loggedIn;

I have a view (Home.html) that has two elements, each with an ng-if attribute.

<div ng-if="home.loggedIn">
    Logged In!
</div>
<div ng-if="!home.loggedIn">
    Not Logged In...
</div>

If I navigate to /Demo/#/Home/true then the first element displays and the second does not. If I navigate to /Demo/#/Home/false then the first element does not display NOR does the second one.

I would expect the !home.loggedIn parameter evaluate to true when the value of loggedIn is, in fact, false.

Any advice here?

回答1:

It is quite obvious that he problem has its root to the fact that routeParams.loggedIn is a string.

So the solution is quite obvious:

// Change that:
this.loggedIn = this.routeParams.loggedIn;

// To this:
this.loggedIn = this.routeParams.loggedIn === 'true';

---

But why the weird behaviour ?
Why work not showing anything when loggedIn is 'false' ?

Well, here is why:

The ngIf directive uses the following toBoolean() function to convert its value to boolean:

function toBoolean(value) {
  if (typeof value === 'function') {
    value = true;
  } else if (value && value.length !== 0) {
    var v = lowercase("" + value);
    value = !(v == 'f' || v == '0' || v == 'false' || v == 'no' || v == 'n' || v == '[]');
  } else {
    value = false;
  }
  return value;
}

If a string is passed to toBoolean() it converts it to lowercase and checks (among other things) if it equals 'false' (in which case it returns false). This is different than the default JavaScript implementation which interprets any non-empty string as true when casting to boolean.

So, let's examine the two cases for both ngIfs:

1. loggedIn === 'true'

   ngIf1 evaluates home.loggedIn --> 'true' (string)
   ngIf1 passes this value through toBoolean()
toBoolean('true') returns true (because it sees a string that can't match with any string considered falsy)
   ngIf1 renders its content

   ngIf2 evaluates !home.loggedIn <=> !'true' --> false (boolean)
   (this happens because any non-empty string happens to evaluate to true)
   ngIf2 passes this value through toBoolean()
toBoolean(false) returns false
   ngIf2 does not render its content

2. loggedIn === 'false'

   ngIf1 evaluates home.loggedIn --> 'false' (string)
   ngIf1 passes this value through toBoolean()
toBoolean('false') returns false (because it sees a string that is considered falsy
   ngIf1 does not render its content

   ngIf2 evaluates !home.loggedIn <=> !'false' --> false (boolean)
   (this happens because any non-empty string happens to evaluate to true)
   ngIf2 passes this value through toBoolean()
toBoolean(false) returns false
   ngIf2 does not render its content

So, this explains the "weird" behaviour (hopefully in an understandable way).

回答2:

Problem is likely home.loggedIn is a string, when passed to ng-if it is probably evaluating and doing the conversion from string to bool to get the value "false" into false. In the expression evaluation before the value is passed through if you have !"false" that is actually false since any string is true, negating it becomes false.