node.js -3- 基本语法

typeof(3.5)
typeof(3)

typeof(true)
typeof(false)

typeof('abc')

var user = {
    first_name: "HTML",
    last_name: "CSS",
    age: 32,
};
var empty = new Object()

console.log(typeof(user))
console.log(typeof(empty))

var a
typeof(a)
typeof(askfalsjdflasf)

function hello(name) {
    console.log("hello " + name);
}

hello('abc')
console.log(typeof(hello))

var a = 10; // 赋值语句
a = 4 * 23 + 188 / 6; // 数学表达式
var c = 7;
var b = (a + c) / 2; // 变量的数学表达式
console.log(a);
console.log(b);

// 比较表达式，如果成立，那么为true,否则为false;
console.log(a > b); 
console.log(a == b);
console.log(a <= b);
console.log(a != b);

//&&所有条件都满足才为真
//||所有条件中只要有一条满足就为真
console.log("&&&&&&&&&");
console.log(false && false);
console.log(false && true);
console.log(true && false);
console.log(true && true);

// 逻辑或
console.log("||||||||||||");
console.log(false || false);
console.log(false || true);
console.log(true || false);
console.log(true || true);

console.log((false && false) || true); // true
console.log(false && (false || true)); // false

// 非
console.log(!true)
console.log(!false)

// 字符串的加法是把各个字符串连接在一起，生成一个新的字符串；
var str = "Hello i am ";
var name = "a girl"
var str_total = (str + name);
console.log(str, name, str_total);

// 字符串与基本的数据类型相加，把这个基本数据类型转成字符串后再生成新的字符串
var num_str = " " + a + b + c + true;
console.log(num_str);

// 简约的写法
a += c;  // a = a + c;
a -= c; //  a = a - c;
a *= c; // a = a * c;
a /= c; // a = a / c;

a ++;  // a = a + 1;
++ a; // a = a + 1;
a --; // a = a - 1;
-- a; // a = a - 1;

if (条件判断)
{
主体
}

> if (a > b)
{
    console.log("a is bigger")
}

if (条件判断)
{
主体
} else
{
主体
}

if (a > b)
{
    console.log("a is bigger")
}
else
{
    console.log("b is bigger")
}

if(条件判断)
{
}
else if {
}
...
else {
}

if (a == 1)
{
    console.log("a is 1")
}
else if(a == 2)
{
    console.log("a is 2")
}
else if(a == 3)
{
    console.log("a is 3")
}
else if(a == 4)
{
    console.log("a is 4")
}
else
{
    console.log("a is " + a)
}

switch(变量) {
case 常量1:
break;
case 常量2:
break;
}

var a = 2

switch (a)
{
    case 1:
    {
        console.log("a is 1")
        break
    }

    case 2:
    {
        console.log("a is 2")
        break
    }

    case 3:
    {
        console.log("a is 3")
        break
    }
    case 4:
    {
        console.log("a is 4")
        break
    }
    default:
    {
        console.log("final a is " + a)
    }
}

while(表达式) { 循环体}

//while(条件成立) {循环体代码}; 
index = 0;
while(index < 10) { // 循环执行了10次；判断一下， 循环条件是否为真，为真就执行，否者就不执行循环体；
    console.log(index);
    index += 2;
}
console.log("out of while");
/*
// 死循环，如果任何时候，这个循环判断条件都为真，那么就是死循环，代码就在循环体里面一直执行；
while(true) { // 死循环，一直执行while语句
    console.log("while call true");
}
*/

for(初始化; 循环条件; 循环迭代) {循环体}

var walk = 0;
var i = 1;
for(walk = 0, i = 1; i <= 10; walk += 2, i ++) {
    console.log(walk);
}

walk = 0;
i = 1;
for(; i <= 10; walk += 2, i ++) {
}
console.log("out for");

// 死循环,如果么有循环判断，默认为true
/*for(;;) {
    console.log("loop for");
}*/

// 省略初始化与循环迭代语句;
i = 6;
for(; i < 10;) {
    console.log("for loop");
    i ++;
}

do {}while(表达式);

// for, while是先判断后执行，do{} while(); 先执行, 再判断是否继续执行循环
i = 0;
do {
    i ++;
    console.log(i);
}while(i < 10);

i = 1;
for(i = 1; i <= 10; i ++) {
    if (i == 8) {
        continue; // 终止本次循环，但是仍然会执行 循环迭代语句 (i ++)；
    }

    console.log("give hime " + i);
}

i = 1;
while(i <= 10) {
    if (i == 8) {
        console.log("while continue");
        i ++;
        continue; // 终止本次循环，回到循环判断，一定要注意循环判断变量，在continue,不要有死循环
    }

    console.log("while give hime " + i);    
    i ++;
}

i = 1;
for(i = 1; i <= 10; i ++) {
    if (i == 8) {
        break; // 终止所有循环,跳出循环代码，但是不会执行 循环迭代语句 (i ++)；
    }

    console.log("give hime " + i);
}
console.log("end_for", i); // i = 8;

i = 1;
while(i <= 10) {
    if (i == 8) {
        console.log("while continue");
        i ++;
        break; // 终止所有循环,跳出循环代码
    }
    console.log("while give hime " + i);    
    i ++;
}
console.log("end while");

var i = "this is a test".indexOf("is");
console.log(i);
->
2

var s = "this is a test string."

console.log(s.substr(19, 3))
console.log(s.substr(-5))
console.log(s.slice(19, 22))
console.log(s.slice(-5))
->
ng.
ring.
ng.
ring.

var s = "a|b|c|d|e|f|g|h".split("|");
console.log(s);
->
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

var s = "       cat   \n\n\n    ". trim();
console.log(s);
->
cat

var s = "cat"
console.log(s.replace('cat', 'dog'))
->
dog