This blog is a personal self-study record. If there is any deficiency, please criticize and correct it.

Version: ES 6

The basic syntax in this chapter refers to the syntax commonly used in JS

Expressions and statements

expression

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

• The value of the console.log expression is the function itself
• What is the value of the console.log(3) expression? Answer: undefined

sentence

• var a = 1 is a statement whose value is undefined

difference

• Expressions generally have values, and statements may or may not
• Statements are generally used to change the environment (declaration, assignment)

• The above two sentences are not absolute, but in general

Case sensitive

• var A and var A are different
• Object and object are different

• Function and function are different

Spaces and carriage returns

As long as it does not affect sentence breaks, most of the spaces are meaningless

• There is no difference between var a = 1 and var a = 1

enter

• Ibid., no difference
• The only place where carriage return cannot be added is after return (if carriage return is added after return, the return value is undefined)

identifier

The variable name is an identifier

rule

• The first character: can be letters, $_ Or Chinese
• The following characters: in addition to the above, they can also be numbers

notes

// Single-Line Comments 

/*
   12
   multiline comment 
   123
*/

Annotation classification

The more notes you write, the better

Bad notes

• Translate the code into Chinese
• Obsolete comments

• Notes to vent dissatisfaction

Good note

• Notes to important information
• Footwell notes

• Why is the code written strangely and what bug s do you encounter

Block block

• Is to wrap the code together with {}

{
  let a = 1
  let b = 2
}

• Often used with if / for / while

if statement

If... Then

grammar

if (condition) {
  // Statement 1
  else {
    // Statement 2
  }
}

• {} can be omitted when there is only one sentence (this is not recommended)

Abnormal condition

• The expression can be very abnormal, such as a = 1

let a = 2;
if (a = 1) {
    console.log("a Yes 1");
}

// Output: a is 1
// Because = is the assignment symbol, the assignment symbol can be used in the expression. If it is judged to be used===

• Statement 1 can be very abnormal, such as nested if else
• Statement 2 can be very abnormal, such as nested if else

• Indent:

a = 1;
if(a === 2)
  console.log('a')
	console.log('a Equal to 2')

// a equals 2

// The above code is equal to:

a = 1;
if(a === 2) {
	console.log('a')
}
console.log('a Equal to 2')

switch statement

if... else... Upgrade

grammar

switch(expression) {
    case value1:
        sentence	// Execute the statement here when the result of expression matches value1
        break;
    case value2:
        sentence	// Execute the statement here when expression matches value2
        break;
    default:
        sentence	// If the expression does not match the value above, execute the statement here
}

Conditional operator?:

Question mark colon expression

Conditional (ternary) operator

Replace if else

grammar

condition ? exprIfTrue : exprIfFalse

Short circuit logic & &||

Instead of if or if else

&&

And short circuit logic

grammar

expr1 && expr2 // If expr1 is true, expr2 is returned; otherwise, expr1 is returned

||

Or short circuit logic

grammar

expr1 || expr2 // If expr1 is true, expr1 is returned; otherwise, expr2 is returned

while loop

When

grammar

while (condition) {
	sentence	// When the condition is true, execute the statement here. After execution, judge whether the condition is true or false again 
}

Initialization variables, judgment conditions, loop body, self increment or self decrement are required
 Example:
var a = 1;		// initialization
while(a <= 10) {	// judge
	console.log(a);	// Circulatory body
      a = a + 1;	// Self increasing
}

for loop

grammar

for(initialization;Judgment conditions;Self increasing or self decreasing) {
  sentence	// Loop body, after executing the statement here, execute self increment or self decrement, then judge, and then execute the statement here
}

break and continue

Exit all loops and skip the current loop

label statement

JavaScript language allows you to have a label in front of the statement, which is equivalent to a locator. It is used to jump to any position of the program. The format of the label is as follows.

label:
  sentence

The label can be any identifier, but it cannot be a reserved word. The statement part can be any statement.

Tags are usually used with break and continue statements to jump out of a specific loop.

foo: {
  console.log(1);
  break foo;
  console.log('This line will not output');
}
console.log(2);
// 1
// 2