Boolean operator parsing

In a programming language, the importance of Boolean operators is comparable to equal operators. If you do not have the ability to test two value relationships, statements such as if...else and loops will be useless.

There are a total of 3 Boolean operators: Non-, with, or

　　1, logical non-
Logic is not used! Indicates that a value of any type that can be applied with ECMAScript, the logical non-operation returns a Boolean value (True/false). The operator first converts its operand to a Boolean value and then deserializes it.

The following describes a set of rules for the Boolean () transformation function.

Data type	The value converted to true	Value converted to False
Boolean	True	False
String	Any non-empty string	"" (empty string)
Number	Any non-0 numeric value (including infinity)	0 and Nan
Object	Any object	Null
Undefined	No	Undefined

A simple representation of the Boolean () transformation function. -----------！！

2. Logic and

The logic and operator are represented by two and number (&&), with two operands.

Logic and operations can be applied to operands of any type, not just Boolean values. In the case where an operand is not a Boolean value, the logic and operation do not necessarily return a Boolean value; At this point, it follows the rules:

1. If the first operand is an object, a second operand is returned;
2. If the second operand is an object, the object is returned only if the value of the first operand evaluates to true;
3. If the two operators are objects, the second operand is returned, followed by the first rule.
4. Returns null if one of the operations is null;
5. If one of the operators is Nan, the Nan is returned;
6. If one of the operators is undefined, the undefined is returned.

Logic and operation are short-circuiting operations, that is, if the first operand can determine the result, then the second operand is no longer evaluated. (Can be understood as an internal two return operation). Therefore, when the 4, 5, 6 rules conflict, the principle of short-circuit operation is followed.

The code is as follows:
var nul = null;
var na = NaN;
var test;
Test = na&&nul;
document.write (test); NaN

The code is as follows:

var nul = null;
var na = NaN;
var test;
Test = nul&&na;
document.write (test); Null

So let's take a look at the summary. && operations are mainly guided by several principles:

1. Short circuit operation principle;
2. The evaluation process will be transformed to generate a copy, but the return value is the original value;

Copy CodeThe code is as follows:

Pseudo code
function && (param1,param2) {
bparam1 = Boolean (param1);
if (!bparam1) return param1;
Bparam2 = Boolean (param2);
return param2;
}
parameter is an object that returns a reference to the value of a pointer, and the function

3. Logical OR

The logic or symbol consists of two vertical bar symbols (| | Said
The logic or symbol is also a short-circuiting operator. The implementation process can refer to the logic and pseudo-code. No more rules are listed here.
The default handling of logic or often used to do parameters, such as Evt = EVT | | window.event;

Boolean operator parsing