Javascript is not easy for beginners either

Here are some tips and pitfalls that Javascript beginners should know. If you're already an expert, brush up on this.

Javascript is just a programming language. How could it possibly go wrong?

1. Have you ever tried to sort a set of numbers?

Javascript's sort() function sorts using alphanumeric (String Unicode code points) by default.

So [1,2,5,10].sort() will output [1, 10, 2, 5].

To sort an array correctly, you can use [1,2,5,10].sort((a, b) => a — b)

Very simple solution, the premise is that you have to know that there is such a pit

2. new Date() is great

new Date() Acceptable:

• No parameters: Return the current time
• One parameter x: Returns January 1, 1970 + x milliseconds. Those who know Unix know why.
• new Date(1, 1, 1) returns 1901, February, 1st/. Because, the first parameter represents 1900 plus 1 year, the second parameter represents the second month of this year (so February) — People with normal brain circuits will start indexing from 1 — , and the third parameter is very Obviously it's the first day of the month, so 1 — sometimes the index does start at 1 — .
• new Date(2016, 1, 1) will not add 2016 to 1900. It only represents 2016.

3. Replace does not "replace"

let s = "bob"
const replaced = s.replace('b', 'l')
replaced === "lob"
s === "bob"

I think this is a good thing because I don't like functions changing their inputs. You should also know that replace will only replace the first matching string:

If you want to replace all matching strings, you can use a regular expression with the /g flag:

"bob".replace(/b/g, 'l') === 'lol' // 替换所有匹配的字符串

4. Pay attention when comparing

// These are ok
'abc' === 'abc' // true
1 === 1         // true
// These are not
[1,2,3] === [1,2,3] // false
{a: 1} === {a: 1}   // false
{} === {}           // false

Reason: [1,2,3] and [1,2,3] are two independent arrays. They just happen to contain the same value. They have different references and cannot be compared with ===.

5. Array is not a primitive data type

typeof {} === 'object'  // true
typeof 'a' === 'string' // true
typeof 1 === number     // true
// But....
typeof [] === 'object'  // true

If you want to know if your variable is an array, you can still use Array.isArray(myVar)

6. Closure

这是一个很有名的面试题：

const Greeters = []
for (var i = 0 ; i < 10 ; i++) {
  Greeters.push(function () { return console.log(i) })
}
Greeters[0]() // 10
Greeters[1]() // 10
Greeters[2]() // 10

你是不是认为它会输出 0, 1, 2… ？ 你知道它为什么不是这样输出的吗? 你会怎样修改让它输出 0, 1, 2… ？

这里有两种可能的解决方法：

用 let 替代 var. Boom. 解决了.

let和var的不同在于作用域。var的作用域是最近的函数块，let的作用域是最近的封闭块，封闭块可以小于函数块（如果不在任何块中，则let和var都是全局的）。（来源）

替代方法: 用 bind:

Greeters.push(console.log.bind(null, i))

还有很多其他方法。这只是我的两个首选

7. 谈到 bind

你认为这个会输出什么？

class Foo {
  constructor (name) {
    this.name = name
  }
  greet () {
    console.log('hello, this is ', this.name)
  }
  someThingAsync () {
    return Promise.resolve()
  }
  asyncGreet () {
    this.someThingAsync()
    .then(this.greet)
  }
}
new Foo('dog').asyncGreet()

如果你认为这个程序会崩溃提示 Cannot read property 'name' of undefined，给你一分。

原因： greet 没有在正确的上下文中运行。同样，这个问题依然有很多解决方案。

我个人喜欢

asyncGreet () {
this.someThingAsync()
.then(this.greet.bind(this))
}

这样可以确保类的实例作为上下文调用greet。

如果你认为greet 不应该在实例上下文之外运行, 你可以在类的constructor中绑定它:

class Foo {
constructor (name) {
this.name = name
this.greet = this.greet.bind(this)
}
}

你还应该知道箭头函数（ => ）可以用来保留上下文。这个方法也可以：

asyncGreet () {
this.someThingAsync()
.then(() => {
this.greet()
})
}

尽管我认为最后一种方法并不优雅。

我很高兴我们解决了这个问题。

总结

祝贺你，你现在可以放心地把你的程序放在互联网上了。甚至运行起来可能都不会出岔子（但是通常会）Cheers \o/

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