Javascript boolean data and = Operator.

Today, I saw an article in blue.ArticleIn JavaScript, the Boolean and = operators are compared and analyzed.

I don't know. I was shocked. I didn't notice such details at ordinary times:

It is estimated that this is one of the important differences between Daniel and cainiao.

When it comes to the boolean type, it is estimated that many people know that the non-zero value in JS is true, so please run the following two lines in the pipelineCode:

1 VaR Itest = 2 ;
2 If (Itest)
3 {
4 Alert ( True );
5 }
6 If (Itest = True )
7 {
8 Alert ( True );
9 }

 

What do you think is the answer?

The answer is: True true

---- Sorry, the answer is true or false, but I believe that the answer is the first one in the eyes of many people.

In order for everyone to remember or know something that is easy to ignore, I will record these differences or details:

1. Simply run a Boolean value. True is true if it is not 0, for example:
 

1 VaR I1 = 1 ;
2 VaR I2 =- 1 ;
3 VaR I3 = 0 ;
4 Alert (Boolean (I1 )); // True
5 Alert (Boolean (I2 )); // True
6 Alert (Boolean (I3 )); // False
7 If (I1 ){} // True
8 If (I2 ){} // True
9 If (I3 ){} // False

 

2. The = operator is not simply to convert non-0 values to true, and vice versa, rather:

1,If type (X) is Boolean, return the result of the comparison tonumber (x) = y.
If type (Y) is Boolean, return the result of the comparison X = tonumber (y ).

That is to say, if an operand is of the bool type, it will be converted to a number for comparison, And the rule is: True-> 1 false-> 0

For example:

1 VaR I1 =- 2 ;
2 VaR I4 = 2 ;
3 VaR I2 = 1 ;
4 VaR I3 = 0 ;
5 If (I1 = True ){} // True-> 1, so 1! =-2 false
6 If (I2 = True ){} // True-> 1, so 1 = 1 true
7 If (I3 = True ){} // This is obvious. False
8 If (I4 = True ) {] // True-> 1, SO 2! = 1 false
9 // // However, the following is different:
10 If (I1 ){} // Equivalent to boolean (-2), not 0, so true
11 If (I2 ){} // It is not 0, so it is true
12 If (I3 ){} // 0, so false
13 If (I4 ){} // Not 0, so true

 

2. If type (X) is number and type (Y) is string, return the result of the comparison X = tonumber (y ).
If type (X) is string and type (Y) is number, return the result of the comparison tonumber (x) = y.

That is to say, when one operand is a string and the other is a number, the string is converted to a number, for example:

1 VaR A =   " 1 " ;
2 VaR B =   " 001 " ;
3 VaR C =   "" ;
4
5 If ( = 1 ){} // A => Number (a) True
6 If (B = 1 ){} // B => Number (B) True
7 If (C = 1 ){} // B => 0 false

 

3. non-null strings are converted to true (whereas null and undefined are false), for example:

1 VaR A = " A " ;
2 VaR B = "" ;
3 If (){} // True
4 If (B ){} // False

 

 

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Here, we think of jquery's $. When this item selects an element, the result returns a jquery object regardless of whether the element exists.

Therefore, you cannot use if ($ ('# id') to determine whether the image is selected. You can use $ ('# id'). length> 0 to judge.

Original article: http://www.blueidea.com/tech/web/2010/7576.asp

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