索引
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Notes
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Exercise
Notes
1、strict mode
strict mode优势：更严格的检查、删除了一些有问题的语言特性。
把 "use strict" 放在文件顶部（必须是文件顶部，放在其它地方就被当成字符串了）或者函数顶部开启js strict模式。
示例1：没启用strict模式，js就会给你悄摸摸地加一个let上去...
function canYouSpotTheProblem() {
"use strict";
for (counter = 0; counter < 10; counter++) {
console.log("Happy happy");
}
}
canYouSpotTheProblem();
// → ReferenceError: counter is not defined
示例2：没启用strict模式，name被悄悄地绑定到一个全局对象上而没报错。
function Person(name) { this.name = name; }
let ferdinand = Person("Ferdinand"); // oops
console.log(name);
// → Ferdinand
console.log(window.name);
// → Ferdinand
// - -- - --修改↓分割线------------------------------------
"use strict";
function Person(name) { this.name = name; }
let ferdinand = Person("Ferdinand"); // forgot new
// → TypeError: Cannot set property 'name' of undefined
PS. 上述情况，用class定义类具有和采用strict模式一样的效果，所以尽量用class定义类。
2、js类型
js在在运行时才会涉及到类型。许多语言在编译期就考虑类型了。
为了减少js类型带来的一些问题，有两个简单的解决方案：
① 注释
// (VillageState, Array) → {direction: string, memory: Array}
function goalOrientedRobot(state, memory) {
// ...
}
可以进行类型检查，并编译成js
3、js测试
采用第三方测试套件或者如下所示：
function test(label, body) {
if (!body()) console.log(`Failed: ${label}`);
}
test("convert Latin text to uppercase", () => {
return "hello".toUpperCase() == "HELLO";
});
test("convert Greek text to uppercase", () => {
return "Χαίρετε".toUpperCase() == "ΧΑΊΡΕΤΕ";
});
test("don't convert case-less characters", () => {
return "مرحبا".toUpperCase() == "مرحبا";
});
4、Debugging
console.log或者debugger关键字：
function add(a, b) {
console.log(`add ${a} and ${b}`);
return a + b;
}
debugger; // 从这一行开始进入浏览器debug模式
let x = add(1, 3);
prompt("只有在开发者模式下，debug模式才生效");
5、Exceptions
异常的一大优势：几乎无耦合地在多个函数中（调用栈）传递，中间函数什么都不需要知道，仅在最外层处理就行了。
function promptDirection(question) {
let result = prompt(question);
if (result.toLowerCase() == "left") return "L";
if (result.toLowerCase() == "right") return "R";
throw new Error("Invalid direction: " + result);
}
function look() {
if (promptDirection("Which way?") == "L") {
return "a house";
} else {
return "two angry bears";
}
}
try {
console.log("You see", look());
} catch (error) {
console.log("Something went wrong: " + error);
}
6、finally
function transfer(from, amount) {
if (accounts[from] < amount) return;
let progress = 0;
try {
accounts[from] -= amount;
progress = 1;
accounts[getAccount()] += amount;
progress = 2;
} finally {
if (progress == 1) {
accounts[from] += amount;
}
}
}
//Writing programs that operate reliably
//even when exceptions pop up in unexpected places
//is hard. Many people simply don’t bother,
//and because exceptions are typically reserved
//for exceptional circumstances, the problem may
//occur so rarely that it is never even noticed.
//Whether that is a good thing or a really bad thing
//depends on how much damage the software will do when it fails.
7、异常分支
js不支持catch分支，而在try-catch的时候，同样会捕获undefined.call等造成的异常。
有些人可能会想只要比对e里面的信息就可以知道哪儿出现问题了，但是异常信息不是一个稳定的东西，一旦抛出的异常信息的表达改变，程序就不会正常工作，更可靠的解决方案如下（自定义空异常，只为了利用class区分，不加任何东西）：
class InputError extends Error {}
function promptDirection(question) {
let result = prompt(question);
if (result.toLowerCase() == "left") return "L";
if (result.toLowerCase() == "right") return "R";
throw new InputError("Invalid direction: " + result);
}
for (;;) {
try {
let dir = promptDirection("Where?");
console.log("You chose ", dir);
break;
} catch (e) {
if (e instanceof InputError) {
console.log("Not a valid direction. Try again.");
} else {
throw e;
}
}
}
Exercises
① Retry
class MultiplicatorUnitFailure extends Error {}
function primitiveMultiply(a, b) {
if(Math.random() < 0.2) {
return a * b;
} else {
throw new MultiplicatorUnitFailure("Klunk");
}
}
function reliableMultiply(a, b) {
let result = undefined;
for (; result == undefined;) {
try {
result = primitiveMultiply(a, b);
} catch(e) {
if (e instanceof MultiplicatorUnitFailure) {
} else {
throw e;
}
}
}
return result;
}
console.log(reliableMultiply(8, 8));
// → 64
————-- - --- -- - - -------—-- - -—-- - -
② The locked box
const box = {
locked: true,
unlock() { this.locked = false; },
lock() { this.locked = true; },
_content: [],
get content() {
if (this.locked) throw new Error("Locked!");
return this._content;
}
};
function withBoxUnlocked(body) {
let locked = box.locked;
if (!locked) {
return body();
}
box.unlock();
try {
return body();
} finally {
box.lock();
}
}
withBoxUnlocked(function() {
box.content.push("gold piece");
});
try {
withBoxUnlocked(function() {
throw new Error("Pirates on the horizon! Abort!");
});
} catch (e) {
console.log("Error raised:", e);
}
console.log(box.locked);
// → true