JavaScript學習 - 基礎(四) - 控制語句/異常處理
阿新 • • 發佈:2018-10-31
控制語句
if-else 語句
<script> //控制語句 //if-else格式: var x = 1 if(x==1){ console.log("this is if") }else if(x>1){ console.log("this is else if") }else { console.log("this else") } </script>
switch語句
<script> //switch語句 var x = 8; switch (x){ case 1:alert("this 1");break; case 2:alert("this 2");break; case 3:alert("this 3");break; case 4:alert("this 4");break; case 5:alert("this 5");break; case 6:alert("this 6");break;case 7:alert("this 7");break; default:alert("nothing") } </script>
for迴圈語句
<script> // for迴圈:for while // 基本格式: for(初始化;條件;增量){語句} for(i=1;i<100;i++){ document.write(i) document.write('<br>') } var x = [1,2,3,4,5,6]for(i=0;i<x.length;i++){ document.write(x[i]) document.write('<br>') } //不建議使用 var x = ['a','b','c','d'] for(i in x){ document.write('this i,輸出:'+i); document.write('<br>'); document.write('this x[i],輸出:'+x[i]); document.write('<br>'); document.write('----------------------'); document.write('<br>'); } // this i,輸出:0 // this x[i],輸出:a // ---------------------- // this i,輸出:1 // this x[i],輸出:b // ---------------------- // this i,輸出:2 // this x[i],輸出:c // ---------------------- // this i,輸出:3 // this x[i],輸出:d // ----------------------
<body>
<p>A</p>
<p>B</p>
<p>C</p>
<p>D</p>
<p>E</p>
<p>F</p>
<p>G</p>
</body>
//不建議使用的原因 var x2 = document.getElementsByTagName('p') for(i in x2) { document.write('this i,輸出:' + i); document.write('<br>'); document.write('this x[i],輸出:' + x2[i]); document.write('<br>'); document.write('----------------------'); document.write('<br>'); } // this i,輸出:0 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:1 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:2 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:3 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:4 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:5 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:6 // this x[i],輸出:[object HTMLParagraphElement] // ---------------------- // this i,輸出:length // this x[i],輸出:7 // ---------------------- // this i,輸出:item // this x[i],輸出:function item() { [native code] } // ---------------------- // this i,輸出:namedItem // this x[i],輸出:function namedItem() { [native code] } </script>
while迴圈語句
<script> //while 迴圈 var i=0;j=0; while(i<201){ j +=i i++; } document.write(j) </script>
異常處理
<script> //異常 //try{主程式}catch(異常){執行}finally{無論是否異常都執行} //主動丟擲異常 throw Error("xxxx") try{ throw Error("報錯了!!!") document.write("OK") } catch (e) { document.write("異常") } finally { document.write("OK and OK") } </script>