Identifier rule

In an application, use variables as symbolic names for values. The name of a variable is also called an identifier, and it needs to follow certain rules.

Must be in letters, underscores (_) Or start with dollar sign ($)
Subsequent characters can also be numbers (0-9).
Because the JavaScript language is case sensitive, letters can be uppercase from "a" to "Z" and lowercase from "a" to "Z".
Most ISO 8859-1 or Unicode encoded characters can be used as identifiers, such as å and ü. You can also use Unicode escape characters as identifiers.

Legal identifier example: Number_hits, temp99, $credit and_ name.

Declare variables

Use the keyword var. For example, var x=42. This syntax can be used to declare local and global variables.

Direct assignment. For example, x =42. Using this form of assignment outside a function produces a global variable. Errors can occur in strict mode. So you should not declare variables in this way.

Use the keyword let. For example, let y=13. This syntax can be used to declare block scoped local variables.

Evaluate Variable

If a variable declared with var or let statements is not assigned an initial value, its value is undefined.

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Block block

Function: package the code together

{
    let a=1
    let b=2
}

Often used with if/for/while

if else statement

if (expression) {Statement 1} else {Statement 2}

{} it can be omitted when the sentence has only one sentence, but it is not recommended to omit.

Writing method

Use a=1 in the expression===

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

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

Switch statement

swith(fruit){
    case "banana":
    //..
    break;
    case "apple":
    //...
    break;
    default:
    break;
    }
}

while statement

while (expression) {statement}

Judge true and false

If the expression is true, execute it, and judge whether it is true or false after execution

If the expression is false, execute the following.

Dead cycle exception

var a=0.1                     //initialization
while(!a==1){              //judge
    console.log(a)         //Circulatory body
    a=+0.1              //increase
    }
}

Because floating point numbers are not accurate

for loop

Is a simple way to write a while loop

for (Statement 1; Expression 2; Statement 3) {
    Circulatory body
}

Statement 1 is used to initialize
Expression to judge
Statement 3 to write growth
The execution order is: statement 1 – expression 2 – loop body – statement 3

If expression 2 does not hold, jump out of the loop directly

special case

Executing the following code will print 5 5

Because the value of delayed print i is 5

for (var i = 0; i < 5; i++){
    setTimeout(() => {
        console.log(i);
    }, 0);
}

The print value depends on when the function is called. We start a delay function setTimeout(fn), a = 5, and print 5.

Is there any way to change that?

If we replace var with let, we will print 011234.

var a = 1;
function fn() {
    console.log(a);
}

break

Exit all current cycles

continue

Exit the cycle closest to it

lable

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

Abbreviation

foo:1
console.log(2)

Question mark colon expression

A?B:C

function max(a,b){
    return a>b ? a:b
}

&&Short circuit logic

A&&b&&c&&d take the first false value or D

consonle&&console.log&&console.log(hi)

||
A | B | C | d take the first true value or D