# Loops: while and for

## The “while” loop

無限迴圈，當判斷式為真，迴圈會持續執行，直到判斷式為錯誤。

```javascript
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。

```javascript
do {
  // loop body
} while (condition);

let i = 0;
do {
  alert( i );
  i++;
} while (i < 3);
```

## The “for” loop

```javascript
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。

```javascript
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。

```javascript
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。

```javascript
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。

```javascript
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` 。

```javascript
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。

```javascript
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!');
```


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