JS learning notes - process control

Process control is to control the structure and order of our code

There are three main structures of process control , Namely Sequential structure 、 Branch and loop structures , These three structures represent the order in which the three codes are executed .

Sequential structure

The sequential structure is in the program The most simple 、 The most basic Process control , It has no specific grammatical structure , Program meeting In the order of the code , Execute sequentially , Most of the code in the program is executed in this way .

Branching structure

In the process of executing code from top to bottom , According to different conditions , Execute different path codes ( Execute code multiple selection - The process of ) , So we can get different results

Branch process control if sentence

if sentence （ Single branch ）

// If the condition holds, execute the code , Otherwise, do nothing 
if ( Conditional expression ) {
    // The code statement executed when the condition is true 
}

if else sentence ( Two branch statements )

// Implementation of conditions if Code inside , Otherwise execution else Code inside 
if ( Conditional expression ) {
    // [ If ] Conditional execution code 
} else {
    // [ otherwise ] Executed code 
}

if else if sentence ( Multi branch statement )

// Suitable for checking multiple conditions .
if ( Conditional expression 1) {
     sentence 1 ;
} else if ( Conditional expression 2) {
     sentence 2 ;
} else if ( Conditional expression 3) {
     sentence 3 ;
} else {
    // Above conditions   It is not established to execute the code here 
}

Ternary expression

Ternary expressions can also make some simple conditional choices . A formula consisting of ternary operators is called a ternary expression

 Conditional expression  ?  expression 1 : expression 2

If the conditional expression turns out to be true , Then return the expression 1 Value . If the conditional expression turns out to be false , Then return the expression 2 Value .

Branch process control switch sentence

switch Statement is also a multi branch statement , It is used to execute different code based on different conditions . When you want to set - Series of specific values when , You can use switch.

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

Using the value of our 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 , Then perform default The words in it .

The value of the expression and case When the values inside match, they are congruent .

switch Statement and if else if The difference between sentences

① In general , They can be replaced by each other .

②swith...case Statements usually deal with case In order to compare the determined values , and f..else.. More flexible sentences , It is often used to judge the range ( Greater than 、 Equal to a certain range )

③switch A conditional statement that executes directly into a program after conditional judgment , More efficient . and ..else There are several conditions for a statement , You have to judge how many times .

Loop structure

stay Js in , There are three main types of circular queries : for loop    while loop     do..while loop .

In the program , A set of repeatedly executed statements is called The loop body , Can you continue to repeat , Depending on the termination condition of the loop . A statement consisting of the body of a loop and its termination conditions , It's called a loop statement .

for loop

for ( Initialize variable ; Conditional expression ; Operation expression ) {
    / / The loop body 
}

for Repeat some code , It's usually related to counting .

Breakpoint debugging

Breakpoint debugging refers to setting a breakpoint on a certain line of the program , During debugging , The program will stop at this line , Then you can debug it step by step , During debugging, you can see the current value of each variable , Are wrong , Debugging to the wrong line of code shows the error , stop .

Breakpoint debugging can help us observe the running process of the program

step ：

         Press in browser F12--> sources --> Find the file you need to debug --> Set breakpoints on one line of the program

        Watch: monitor , adopt watch You can monitor the value of variables , Very often .

        F11: Step by step , Let the program execute line by line , This is the time , Observe watch The change in the value of a variable in .

A nested loop

Loop nesting means Define the syntax structure of a circular statement in a circular statement , For example, in for In loop statement , You can nest another for loop , In this way for Circular statements we call double for loop .

for ( The initialization variable of the outer layer ; The conditional expression of the outer layer ; The operation expression of the outer layer ) {
    for ( in   Initialization variables of layer ; The conditional expression of the inner layer ; Inner layer operation expression ) {
        //  Execute statement ;
    }
}

while loop

while Statement can be used if the conditional expression is true , Loop executes a specified piece of code , Until the expression is not true .

while ( Conditional expression ) {
    // Loop body code 
}

Execution steps ：

① First execute the conditional expression , If the result is true , The loop body code is executed ; If false , Then exit the loop , Execute the following code

② Execute the loop body code .

③ After the loop body code is executed , The program will continue to judge the execution condition expression , If the condition is still true , The loop body will continue to execute , To cycle condition is false when , The whole cycle will end .

do while loop

do.... while The sentence is actually while A variant of the statement . The loop executes a block of code first , Then judge the conditional expression , If the condition is true , The loop body will be executed repeatedly , Otherwise exit the loop .

do {
    // The loop body 
} while ( Conditional expression )

continue keyword

continue Keywords are used for Immediately jump out of this cycle , So let's go to the next loop ( In this cycle continue After that, the code will be executed once less ) .

break keyword

break Keywords are used for Jump out of the cycle immediately ( The loop ends ).