Summary of the usage of process control statements in JavaScript programs

Conditional judgment statement

1.if statement

 if (condition) statement1 else statement2

The condition condition can be any expression, and the evaluation result of this expression is not necessarily a Boolean value. ECMAScript will automatically call the Boolean() conversion function to convert the result of this expression into a Boolean value. If the result of evaluating condition is true, statement1 is executed, and if the result of evaluating condition is false, statement2 is executed. These two statements can be either a line of code or a block of code (recommended).

 if (condition1) statement1 else if(condition2) statement2 else statement3

2. Switch statement

 switch(expression) { case value: statement break; case value: statement break; case value: statement break; default: statement}

The meaning of each case in the switch statement is: "If the expression is equal to this value, then the following statement statement is executed". And break will cause the code to execute and jump out of the switch statement. If the break keyword is omitted, it will cause the next case to continue to be executed after the current case is executed. The final default keyword is used to execute maneuver code (and therefore, it is equivalent to an else statement) when the expression does not match any of the previous situations.

Example 1

 var num = 25;switch (true) { case num < 0: alert("Less than 0"); break; case num >= 0 && num <= 10: alert("Between 0 and 10"); break; case num > 10 && num <= 20: alert("Between 10 and 20"); break; default: alert("More than 20");}

Note: The switch statement uses congruent operators when comparing values, so no type conversion occurs.

Loop statement

After testing loop statements.

The code in the loop body is executed at least once before evaluating the expression.

1.do-while statement

 do { statement} while (expression);

Example 2

 var i = 0; do { i += 2; } while (i < 10); alert(i);//10

Pre-test loop statement

2.while statement

 while(expression) statement

Example 3

 var i = 0; while(i < 10) { i += 2; } alert(i);//10

3.for statement

It has the ability to initialize variables before executing the loop and define the code to be executed after the loop is executed.

Example 4

 for(var i = 0;i < 10;i++) { alert(i);}

Iterative statements

for-in statement

The for-in statement is an accurate iterative statement that can be used to enumerate the properties of an object.

 for (var prop in window) { console.log(prop); }

The properties of ECMAScript objects have no order. Therefore, the attribute names output through the for-in loop are unpredictable. Specifically, all attributes will be returned once, and the order of SLR destruction may vary depending on the browser. It is recommended to check and confirm that the value of the object is not null or unedfined before use.

Control execution statement

break statement

Exit the loop immediately

Example 5

 var num = 0; for(var i = 1;i < 10;i++) { if(i % 5 == 0) { break; } num++; } alert(num);//4

Continue statement

Exit the currently executing loop code and start continuing the loop.

 var num = 0; for(var i = 1;i < 10;i++) { if(i % 5 == 0) { continue; } num++; } alert(num);//8

Return statement

The return value can be specified or not (returned undefined at this time).

1. Stop and exit the function immediately after the return statement, and the statement after the return will never be executed.

2. The return statement can only appear in the function body. If it is not, it will automatically report a syntax error.