JavaScript Lesson 4: Making Decisions [II]

Course 4 of JavaScript 0 basics Making Decisions (Part 2)

The fourth lesson is divided into seven sections:

(This article is long and smelly, but it is extremely easy to understand. It is only suitable for beginners of JavaScript, especially those with no foundation)

Translated by: Dan Xiaoru rat

Author's website: http://www.haoxiaoru.info/

Course review:

JavaScript Lesson 1: Variable Variables

JavaScript Lesson 2: Operator Operators

JavaScript Lesson 3: Making Decisions [I]

4.1 Non-boolean result

In the previous tutorial, we can see how JavaScript converts numbers and strings in operations. Similarly, when a Boolean value is required for execution, JavaScript converts other data types to a Boolean value.

Most values are treated as true, with only a few exceptions. For example, a numeric variable with a value of 0 and a variable with a string containing a null string. As follows:

If (1) {... true
If (0) {... false
If ("") {... false
If ("") {... true
If {"0") {... true
If (undefined) {... false
If (null) {... false

The last two variables are undefined and null, which are often used for feature sensing. That is to say, we can hide some JavaScript code for browsers that do not support special features, so that JavaScript code can be correctly executed in multiple browsers.

4.2 multi-condition test

Arithmetic Operations usually include not only two numbers and an operator. In the same case, when the if condition is determined, it will not only contain a binary comparison result. In combination with multiple numeric operations, you only need to add Arithmetic Operators of the same type, such as a + B + c. The results of combining multiple comparative operations are more complex. We do not need to judge such situations as a = B = c, but the situation where both conditions are true or not, and only one condition is true.

 

In JavaScript, you can use logical operators to compare the results of a combination of operations. There are three logical operators: &, or |, not !. If you are not familiar with the logical operation results, see the following table.

Logical "and" Operator

&&	True	False
True	True	False
False	False	False

Logical "or" Operator

|	True	False
True	True	True
False	True	False

Logical "Non" Operator

!
True	False
False	True

• The logical & operator is true only when both conditions are true. This means that when the condition on the left is false, you do not need to judge the right. The result must be false.
• Logic | Operator. If one condition is true, the result is true. This means that when the condition on the left is true, you do not need to judge the right. The result must be true.
• Logic! Operator. Generally, you can convert the result of a Boolean variable.

Reminder: do not forget that & is composed of two &, | is composed of two |. If you write only one bitwise operator, the result is quite different.

Priority: Arithmetic Operator> comparison operator> logical operator. The same operator is prioritized from left to right. If the logical operator can produce results without completing the operation, it is not necessary to complete the operation.

In any case, you can use parentheses () to change the priority order (). Sometimes, to make reading easier, you can also use parentheses (), which do not change the priority order but are easy to read.

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