while (condition) {
// code
// so-called "loop body"
}
let i = 0;
while (i < 3) { // shows 0, then 1, then 2
alert( i );
i++;
}
// single line omit {...}
let i = 3;
while (i) alert(i--);
The “do…while” loop
迴圈至少執行一次。
do {
// loop body
} while (condition);
let i = 0;
do {
alert( i );
i++;
} while (i < 3);
The “for” loop
for (begin; condition; step) {
// ... loop body ...
}
for (let i = 0; i < 3; i++) { // shows 0, then 1, then 2
alert(i);
}
變數被宣告在迴圈裡面，只能在迴圈內使用，除非使用變數為全域變數。
for (let i = 0; i < 3; i++) {
alert(i); // 0, 1, 2
}
alert(i); // error, no such variable
// global variable
let i = 0;
for (i = 0; i < 3; i++) { // use an existing variable
alert(i); // 0, 1, 2
}
alert(i); // 3, visible, because declared outside of the loop
for 迴圈可以忽略任何部分，但；要存在不然會有 syntax error。
let i = 0; // we have i already declared and assigned
for (; i < 3; i++) { // no need for "begin"
alert( i ); // 0, 1, 2
}
let i = 0;
for (; i < 3;) {
alert( i++ );
}
Breaking the loop
用 break 可以離開迴圈。
let sum = 0;
while (true) {
let value = +prompt("Enter a number", '');
if (!value) break; // (*)
sum += value;
}
alert( 'Sum: ' + sum );
Continue to the next iteration
用 continue 可以停止當次運算，開始下次運算。
for (let i = 0; i < 10; i++) {
// if true, skip the remaining part of the body
if (i % 2 == 0) continue;
alert(i); // 1, then 3, 5, 7, 9
}
// same, side-effect less readability
for (let i = 0; i < 10; i++) {
if (i % 2) {
alert( i ); // if there are multi lines to execute
}
}
在 ? operator 不能使用 break / continue 。
if (i > 5) { // worked
alert(i);
} else {
continue;
}
// same
(i > 5) ? alert(i) : continue; // continue isn't allowed here
Labels for break/continue
label 停止巢狀迴圈，用 break 只能停止一個迴圈。
outer: for (let i = 0; i < 3; i++) {
for (let j = 0; j < 3; j++) {
let input = prompt(`Value at coords (${i},${j})`, '');
// if an empty string or canceled, then break out of both loops
if (!input) break outer; // (*)
// do something with the value...
}
}
alert('Done!');