如何缩短这个条件？

Question

if (!this.initialized || (typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId)) {
    //Stuff
}

I want to execute code if the this.initialized flag is false or this.activeId value isn't the same as options.qualification_id . I have to check if this.activeId is defined or not because Javascript will return true in the case of undefined !== this.activeId . The above is the conditional I've come up with, but I hate how long it is. What would be the shortest way to write it?

Answer 1

如果this.activeId在定义时的有效值都是真值（例如它不能是0 、 false或null ），您可以使用：

if (!this.initialized || this.activeId && options.qualification_id != this.activeId)

Answer 2

Let's see what you can do to shorten this

(!this.initialized || (typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId))

Well first, you can remove the inner parenthesis. According to the operator precedence table, logical AND has a "higher precedence" than logial OR. In other words the parenthesis will be added for you by the interpreter. So, your condition becomes:

(!this.initialized || typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId)

Not much better. Another thing you can do is take advantage of JS type conversion. undefined values are falsey (are evaluated to false ), your condition can become:

(!this.initialized || this.activeId && options.qualification_id !== this.activeId)

However, this may insert a silent bug since an activeId of 0 will also be evaluated to false . You can solves this using the in operator which checks to see if the property exists on the object or on one of its prototypes (To make sure the property doesn't come from one of its prototypes you can use the hasOwnProperty method instead).

(!this.initialized || 'activeId' in this && options.qualification_id !== this.activeId).

Now to be honest, this doesn't look so much better to me. As a rule, whenever the line becomes too long, just split it in multiple lines to increase readability.

var qualificationIsDifferent = options.qualification_id !== this.activeId;
var isNotActive = this.hasOwnProperty('activeId') && qualificationIsDifferent;
var shouldDoStuff = !this.initialized || isNotActive;

if ( shouldDoStuff ) { ... }

Note that by using the above approach you lose some of the benefits of the short-circuit evaluation . It really depends on your goal, sometimes more verbose means more readable.