1. What are expressions and statements

1.1 expression

  1. The 1+2 expression has the value 3
  2. The value of the add(1,2) expression is the return value of the function

1.2 the statement

Var a = 1 is a statement

1.3 Differences between the two

  1. Expressions generally have values, and statements may or may not have values
  2. Statements generally change the environment (declaration, assignment)

2. Rules for identifiers

2.1 Naming Rules

  1. The first character can be a Unicode letter or $or _ or Chinese
  2. The following characters, in addition to the above, can also be numbers

2.2 pay attention to

  1. At most, there are two underscores, and then I can’t count them
  2. Prefer to use Chinese, do not use Chinese pinyin, because programmers pinyin may be different

3. If the else statement

3.1 grammar

If (expression){statement 1}else{statement 2}

  • If the expression is true, statement 1 is executed; If the expression is false, statement 2 is executed

{} can be omitted only if the statement is one line long, but this is not recommended

3.2 Recommended writing method

  1. The most recommended way to write it

If (expression){statement}else if(expression){statement}else{statement}

  1. This is recommended

Funtion fn(){if(expression){return expression}if expression){return expression} return expression}

4. While for statement

4.1 while

While (expression){statement}

  • Judge the expression is true or false, when the expression is true, execute the statement, execute the expression is true or false; When the expression is false, we jump out of the loop and execute the statement following the while
  • For example,

4.2 the for

For (statements 1; Expression is 2; Statement 3){body of loop}

  • Execute statement 1 once, then check expression 2, if true, execute the body of the loop, then execute statement 3, then check expression 2; If false, execute the statement following the for loop
  • For example,

5.break continue

5.1 break

Break Exits all loops except one

5.2 the continue

Continue Exits a loop

6.label

Label: statement