Summary of JavaScript basic execution flow control syntax examples

Conditional judgment statement

1.if statement

if (condition) statement1 else statement2

condition The condition can be any expression, and the expression does not necessarily evaluate to 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. 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
}

Each of the switch statements The meaning of case is: "If the expression is equal to this value, execute the following statement statement." Break will cause code execution to jump out of the switch statement. If the break keyword is omitted, the next case will continue to be executed after the current case is executed. The final default keyword is used to execute the maneuver code (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 the congruence operator when comparing values, so it will not happen Type conversion.

Loop statement
Test the loop statement after.
The code within the loop body is executed at least once before the expression is evaluated.

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.
Example 4

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

Iteration statement
for-in statementfor-in statement is A precise iteration 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 are not ordered. 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 statementExit 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 then Start continuing the cycle.

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

return statement

You can specify a return value or not (return undefined in this case).
1. Stop and exit the function immediately after the return statement, and the statements after return will never be executed 2. The return statement can only appear within the function body, if not, a syntax error will be automatically reported

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