3 operators you need to pay attention to in Javascript_javascript tips

You are used to writing in C#, so you will think that what kind of operators should do what kind of operations, but one day your habits are subverted by other languages, I wonder if you have a strong curiosity

Xin, just in js, this habit of mine was subverted. Let’s take a look at which operators subverted my three views.

One: == operator

　The reason why the == operator can be subverted can be seen from the following examples.

<1> "10"==10 ?

If you put this in C#, the compiler will tell you unceremoniously, you bastard, the types are different, you are just a fool. . . But what happens in JS?

From the picture above, you can see that whether you are curious or not, the answer is there. Someone may ask whether 10 is converted into "10" or "10" is converted into 10. So this is also

This is what I always complain about. If it is C#, you can also see how it is handled in IL, but in JS you can’t see anything, you can only listen to one-sided words in the textbook, and you cannot see what you see.

So in addition to remembering it, I can only say that the string "10" was converted into 10, and then compared with integer.

<2>true==1 ?

This question can be understood after a little thought. In fact, in C#'s IL, true and false are converted into 1 and 0 accordingly, so js will convert true into 1 when judging, and then perform plastic comparison. This

I don’t think this phenomenon is very strange, just remember it.

<3> {valueOf:function(){return "10"}}==10?

This question is also quite strange. Can objects be compared with int types? But it can really be done in JS. The principle is like this. If an object is compared with int/string,

JS internally calls the valueOf method first, which is to digitize the object. In fact, the fun part here is that our custom valueOf overrides the valueOf method of the parent class, so the above

The example of

is to judge "10" == 10?.

One more PS here. If the valueOf method is not defined in your class, the js internal engine will look for the toString() method again and execute it if there is one.

I wonder if you feel a little flustered and confused when you come into contact with these new usages? If you are afraid of potential bugs in your program, then to be on the safe side,

Convert it to the same data type for comparison. Of course, there is also a === operator in js, which can be said to be the closest to the logical operator in C#. The extra "=" is in

Based on "==", determine whether the types are equal? For example:

2: && and || operators

These two operators are also quite strange. In our thinking habits, both sides of these two operators should be of bool type, but in JS these definitions will be completely subverted. It is these new rules that So we can complete many new tricks. For example, in the source code of jquery, you can find traces of this everywhere.

From the picture we see this sentence, bup=b && b.parentNode, can you understand the meaning of this sentence? In fact, it means to first determine whether b exists. If b does not exist, then

Maybe b is undefined, null, 0 or NaN. If b exists, then m.parentNode is returned. It’s so fun. If you use C#, you will need a few if conditions,

Let’s simplify the code, and then look at the || operation. This can be described as overflowing in the source code, but at first glance, it looks like the nullable operator in C#, so it feels more familiar

Increase, let’s take ret=results||[] as an example. If results has a value, then ret=results. If results is null, undefined, NaN or 0, then result=[], then

Such willfulness saves programmers a lot of if judgments. Finally, I would like to add that it is actually just like the IL in C#, but the bottom layer of JS makes the if judgments for us.