template string

模板字符串---反引号

定义

       模板字符串使用反引号(`)来代替普通字符串中的用双引号和单引号.模板字符串可以包含特定语法( $ {expression} ）的占位符.占位符中的表达式和周围的文本会一起传递给一个默认函数,该函数负责将所有的部分连接起来,如果一个模板字符串由表达式开头,则该字符串被称为带标签的模板字符串,该表达式通常是一个函数,它会在模板字符串处理后被调用,在输出最终结果前,你都可以通过该函数来对模板字符串进行操作处理.在模版字符串内使用反引号(`）时,需要在它前面加转义符(\).

语法

1. Embed the string directly

2. Embed a multiline string

3. 插入表达式

4. 带标签的模板字符串

模板标签

       标签模板其实不是模板,而是函数调用的一种特殊形式.“标签”指的就是函数,紧跟在后面的模板字符串就是它的参数. 但是,如果模板字符里面有变量,就不是简单的调用了,而是会将模板字符串先处理成多个参数,再调用函数.

toString方法

 

函数中有toString方法,所以将1into a string for execution

eval函数

 

eval：will be included in the incoming stringJavaScript代码执行,If the input is not a string, it will be returned unchanged

evalVariables will be found under global variables,If the string is not defined globally, it will report undefined-->  undefined

 

 Why is the output an empty array？

We understand through a few exampleseval、反引号、模板字符串

<script>eval`${prompt(1)}`</script>

        The popup function can be executed, function executed inside,The function is executed first from the template string（模板字符串中可以调用函数）,再由evalWhether the execution can pass parameters.但是evalOnly the first value can be accepted,However, the title第一个参数is made in the template string执行函数${}An array of left and right strings combined,由于${}There is no value left or right,So it returns an array of two empty characters.evalOnly the previous value is accepted in the question-->空数组,The return value passed in the following function cannot be returned,So the popup window will not return no matter what value is entered.evalOutput the empty array as is.  

<script>eval`${`prompt(1)`}`</script>

       The popup function cannot be executed,evalThe first one received is an empty array,The second is the function that prints the template string,Because the backticks used are equivalent to will{}中的字符串prompt(1)This function prints out and passes it toeval,但是evalOnly the first value is accepted,So the second function is not accepted,也就没有执行prompt这个函数.

<script>eval`${`prompt\x281\x29`}`</script>

Same as the second example,The popup function cannot be executed,But can return empty array. Although the parentheses are coded in the question,But the effect is the same as without encoding,Backticks willpromptThe function is printed and passed toeval但是evalOnly the first parameter is accepted-->空数组,所以prompt函数没有执行.

<script>eval.call`${`prompt\x281\x29`}`</script>

因为eval是一个危险函数,So browsers are righteval有限制,换一个浏览器试试

 

         The popup function can be executed,And can output the content entered in the pop-up window.由于callThe first value in is an empty array pointernull,所以eval被call改变指向后,指向全局（window）,第二个值为prompt(1)字符串,这时evalcan be globally（window）Find this function in and execute it.The execution function returns what the function returns

cell函数的作用：可以改变this的指向

• 第一个参数this,如果第一个参数为null或为underfined,就将evalbound to a global variable

• The second is the passed parameter

<script>eval.call`${prompt(1)}`</script>

       The popup function is executed ,The template string will execute the function first,After the function is executed, an undefined string is entered,evalWhat is accepted is this string,这时evalThe string cannot be found globally,will return the string undefined.When the input string is defined, it will return the defined content.

<script>eval.call`prompt(1)`</script>

       The popup function is not executed,There is no expression in it,call方法会将eval直接指向prompt,因为window对evalThis dangerous function has settings,evalCalled for an alias,官方JS都强制eval指向全局,所以evalcannot be pointed toprompt,At this point there is no second parameter,所以eval什么都没有执行.

        evalThere are various forms of alias invocation,As long as it is not called directly,All are alias calls,Because the engine can only telleval()One form of this is direct invocation,will be pointed towindow.

eval.call`$alert(1)`

       The reason why the value obtained is empty：alert(1)The returned value is empty,所以evalThe last received value is also empty,得到的值就是undefined.