操作流程

条件

if 语句

if (condition1) {
  statement1;
} else if (condition2) {
  statement2;
} else {
  statement3;
}

switch 语句

switch (i) {
  case 25:
  /* falls through */
  case 35:
    console.log("25 or 35");
    break;
  default:
    console.log("Other");
}

falls through

• switch 语句命中 case 后, 继续执行下一个 case;
• 通过 break 控制;

循环

while 语句

while (condition) {
  statement;
}

do-while 语句

do {
  statement;
} while (expression);

for 语句

• 三部分可选;

for (initialization; expression; incrementExpression) {
  statement;
}

等效的 while 语句

initialization;
while (expression) {
  console.log(i);
  statement;
  incrementExpression;
}

for-in 语句

• 遍历对象原型链所有可枚举属性;

for (property in expression) {
  statement;
}

for-of 语句

• 遍历对象原型链所有可迭代属性;

for (property of expression) {
  statement;
}

for...in 和 for...of 的区别

• for in 一般用来遍历对象的 key;
• for of 一般用来遍历数组的 value;

跳转

Labeled 语句

• 标识 statement, 用于 continue 和 break;

start: for (let i = 0; i < count; i++) {
  console.log(i);
}

break 和 continue 语句

break

• 跳出最近的 loop 或 switch;
• 跳出指定 labeled statement;

let num = 0;
outermost: for (let i = 0; i < 10; i++) {
  for (let j = 0; j < 10; j++) {
    if (i == 5 && j == 5) {
      break outermost;
    }
    num++;
  }
}
console.log(num); // 55

continue

• break 的跳过版本;

let num = 0;
outermost: for (let i = 0; i < 10; i++) {
  for (let j = 0; j < 10; j++) {
    if (i == 5 && j == 5) {
      continue outermost;
    }
    num++;
  }
}
console.log(num); // 95

with 语句

• 简化多次调用同一对象的操作;
• 并将 with 对象对应的 execution context 移动至 scope chain 最前端;
• strict mode 禁止使用;

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

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