[JavaScript知识库]javascript逻辑语句

if语句

   		if(true){
            console.log('yes');		//yes
        }else{
            console.log('no');
        }

while语句

• 循环一次i+2，i<10条件满足循环继续

        let i = 0
        while(i < 10){
            i += 2;
        }
        console.log(i)  //10

do-while语句

        let i = 0;
        do{
            i += 2;
        }while(i < 10)
        console.log(i)  //10

for语句

       for (let i = 0; i < 5; i++) {       
            console.log(i);         //0 1 2 3 4 
        }

for-in语句

     	for(const propName in window){
            document.write(propName);
        }

for-of语句

      	for (const el of [2,4,6,8]) {
            document.write(el);
        }

break和continue语句

• break直接跳出循环

       	let num = 0;
        for (let i = 0; i < 5; i++) {
            if(i == 2) break;
            num++;
        }
        console.log(num);       //2

• continue立即退出这次循环，继续下次循环

        let num = 0;
        for (let i = 0; i < 5; i++) {
            if(i == 2) continue;
            num++;
        }
        console.log(num);   //4

with语句（不使用！！！）

		let qs = location.search.substring(1);
        let url = location.href();

等价于

        with(location){
            let qs = search.substring(1);
            let url = href;
        }

with语句影响性能难于调试，不推荐使用！！

swith语句

     	let i = 66;
        switch(i){
            case 11:
                console.log('11');
            case 66:
                console.log('66');      //66
            case 77:
                console.log('77');      //77
        }

 		let i = 66;
        switch(i){
            case 11:
                console.log('11');
                break;
            case 66:
                console.log('66');  
                break;                      //66
            case 77:
                console.log('77'); 
                break;                      
        }

为了不必要的判断，最好在每个条件后面都加上break语句。