JS process control

Process control

During program execution , various Code execution order Yes The result of the procedure It's influential , Many times we have to Control the execution order of the code Come on Realization What we want function

Simple understanding ： Process control controls the structural order in which the code is executed

Process control has three structures , Sequential structure , Branching structure , Loop structure , The three structures represent three code execution sequences

Sequential structure

According to the code Order , Execute sequentially

Branching structure

During execution from top to bottom , according to Different conditions , perform Different path codes , obtain Different results

• if sentence
• switch sentence

if ( Conditional expression ) {
    // Execute statement 
}

If if The result of the conditional expression is true , Execute... In braces , If if The conditional expression is false , Do not execute the , perform if Code after statement

if else sentence

 if The words in it 1 and else The words in it 2 Finally, only one statement can execute

else Followed directly by braces

if else if （ Multi branch statement ）

if （ Conditional expression 1） {
     Execute statement 1
} else if （ Conditional expression 2） {
     Execute statement 2
} else if （ Conditional expression 3） {
     Execute statement 3
} else {
     Last statement 
}

  Ternary expression

Yes Ternary operator The formula is a ternary expression

 Conditional expression  ？  expression 1 ：  expression 2

Execution ideas ： If the conditional expression is true , Then return the expression 1 Value , If the conditional expression is false , Then return the expression 2 Value

switch sentence

It is also a multi branch statement , Used to execute different code based on different conditions , When you want to set a series of specific value options for variables , Use switch

switch （ expression ） {
    case value1：
             Execute statement 1;
            break;
    case value2：
             Execute statement 2;
            break;
        ... 
        default：
             Last statement ;
}

Execution ideas ： Use the value of the expression and case The following option values match , If match up , It's time to do it case The words in it , If none of them match , execute default The words in it

Be careful ：

• In development , Expressions are often written as variables
• num The value of and case When matching inside, it is congruent , The value must be consistent with the data type
• If at present case There's no break, Will not quit switch, Is to continue executing the code , Whether it can match or not

switch Statement and if else if The difference between

1. General situation , Can replace each other
2. switch case Usually dealing with case Compare the situation of certain value ,if else if More flexible , It is often used to judge the range
3. switch Statement to directly execute the conditional statement of the program after judgment , More efficient ,if else There are several conditions , You have to judge how many times
4. When there are fewer branches if else More efficient
5. When there are many branches ,switch More efficient