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?

Javascript'ssort()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

new Date()Acceptable:

• No parameters: Return the current time
• One parameterx: 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.

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 thatreplacewill only replace the first matching string:

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

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

// 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===.

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 useArray.isArray(myVar)

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

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。

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/

The above is the detailed content of Javascript is not easy for beginners either. For more information, please follow other related articles on the PHP Chinese website!