This introduces basic syntax for expressions, statements, identifiers, if else and while and for loops.

Expressions, statements, and identifiers

1+2// The expression has a value of 3;
add(1.2) The value of the expression is the return value of the function
console.log // The expression is the function itself
Copy the code

Expression – refers to a calculation to get a return value.

Statement – An action to complete a task. For example, var a = 1 + 3 is an assignment statement.

The difference between expressions and statements:

  • Expressions generally have values; statements may or may not
  • Statements typically change the environment (declaration, assignment)
  • The first two are not absolute

Identifier rule

  • The first letter can be a Unicode letter or $or _ or Chinese.
  • The following characters, in addition to the above types, can also be numbers.
  • JS case sensitive
  • Spaces and carriage returns are mostly meaningless, but do not return with carriage returns
  • Note: there are // and /* */

2. Conditional statements

If syntax: if(expression){statement 1}else{statement 2}

When the expression meets the judgment condition, statement 1 is executed. Otherwise, statement 2 is executed.

A switch statement:

/*swith (variable) {case "value" : ······; break; } * /
switch (fruit) {
  case "banana":
    // ...
    break;
  case "apple":
    // ...
    break;
  default:
    // ...
}
Copy the code

In switch statements, when the variable is equal to a value of case, the following statement is executed, but each case must be terminated with a break, otherwise the rest of the statement continues.

Question mark colon expression: expression 1? Expression 2: Expression 3

Expression 2 is executed when expression 1 is true; Otherwise, expression 3 is executed.

&& Short circuit logic:

Expression 1 && expression 2 — If expression 1 is true, expression 2 is executed

A && B && C && D — take the first false value or D.

Because IE doesn’t have console, use console && console.log && console.log(‘hi’)

Short-circuit logic: | | | a | b equivalent to the if (! a){b}

C | A | B | | | | D – the first true value or D.

Three, circulation

While loop: while (expression) {statement}

If the expression is true, the statement is executed, and then the expression is checked again. When the expression is false, the current loop ends. Similarly, do… while.

For loop: for(statement 1; Expression is 2; Statement 3){body of loop}

Statement 1 — initialize; Expression 2 — Judgment; Statement 3 — increment;

When expression 2 is judged to be true, the loop body is executed and statement 3 is executed to evaluate expression 2 again.

When expression 2 is judged to be false, the loop is ended.

Break and continue

Break — Exits the current loop.

Continue — Skips the current loop.

The label tag

The statement is preceded by a label, which acts as a locator and is used to jump to any point in the program. The label has the following format

Label: statementCopy the code

Tags can also be used to jump out of code blocks, for example

foo: {  / / foo label
  console.log(1);
  break foo;
  console.log('This line will not print');
}
console.log(2);
Copy the code

The above code breaks foo to exit the block, and continue is used similarly.

{
 a:1
}
Copy the code

A: is the statement label, and 1 is the statement body.

The basic syntax of JavaScript – netpath