Detailed explanation of the use of eval in JavaScript

In our daily development work, I believe many friends are familiar with the use of eval in JavaScript, but for those who are just new to JavaScript, the usage of eval is a bit unfamiliar. Today we will give you a detailed explanation of the use of eval in JavaScript!

The eval function receives a parameter s. If s is not a string, it returns s directly. Otherwise, execute the s statement. If the execution result of the s statement is a value, this value is returned, otherwise undefined is returned.
It is important to note that the object declaration syntax "{}" cannot return a value. It needs to be enclosed in parentheses to return a value. A simple example is as follows:

The code is as follows:

var code1='"a" + 2'; //表达式 
varcode2='{a:2}'; //语句 
alert(eval(code1)); //->'a2' 
alert(eval(code2)); //->undefined 
alert(eval('(' + code2 + ')')); //->[object Object]

You can see that for object declaration statements, they are only executed and cannot return a value. In order to return the commonly used "{}" object declaration statement, it must be enclosed in parentheses to convert it into an expression before its value can be returned. This is also one of the basic principles of using JSON for Ajax development. It can be clearly seen in the example that the second alert statement outputs undefined, while the third one after adding parentheses outputs the object represented by the statement.
Now let’s talk about the focus of this article, how to execute global code within a function. In order to illustrate this problem, let's look at an example first:

The code is as follows:

var s='global'; //定义一个全局变量 
function demo1(){ 
eval('var s="local"'); 
} 
demo1(); 
alert(s); //->global

It's easy to understand. The above demo1 function is equivalent to: function demo1(){var s=' local';}, which defines a local variable s.
So it is not surprising that the final output is global. After all, everyone can clearly distinguish between local variables and global variables.
If you look carefully, you can find the characteristics of the eval function. It is always executed in the context variable space (also called: package, closure) in which it is called. This is true whether it is a variable definition or a function definition, so as follows The code will generate an error of undefined function:

The code is as follows:

var s='function test(){return 1;}'; //一个函数定义语句 
function demo2(){ 
eval(s); 
} 
demo2(); 
alert(test()); //->error:test is not defined

This is because the test function is defined in the local space and can be accessed within the demo2 function, but cannot be accessed from outside. .
In actual Ajax development, sometimes we need to dynamically obtain code from the server for execution to alleviate the problem of loading too much code at one time, or some codes are generated by Javascript itself and we hope to use the eval function. its execution.
But such dynamic code acquisition work is usually completed within the function, for example:

The code is as follows:

function loadCode(){ 
varcode=getCode(); 
eval(code); 
}

It can be seen that eval cannot be executed in the global space, which means It has brought a lot of problems to development, and I have seen many people frustrated by it.
But now I finally found a solution. Hey, it can be compatible with IE and Firefox at the same time. The method is as follows:

The code is as follows:

var X2={} //my namespace:) 
X2.Eval=function(code){ 
if(!!(window.attachEvent && !window.opera)){ 
//ie 
execScript(code); 
}else{ 
//not ie 
window.eval(code); 
} 
}

Now if you want to use it inside the function To define global code, you can call the X2.eval_r(code) method. An example is as follows:

The code is as follows:

var s='global'; 
function demo3(){ 
X2.Eval('var s="local"'); 
} 
demo3(); 
alert(s); //->'local'

It can be seen that the global variable is redefined in the demo3 function s=”local”.
It should be noted that X2.Eval does not return a value. If you want to evaluate an expression, you should still use the system's eval function. X2.Eval is designed to be used only for global code definition.
In fact, after seeing this, some people may feel that the problem is too easy to solve, haha, but finding this method requires some luck and skills:
(1) For IE browser, Such a function is provided by default: execScript, which is used to execute code in the global space, but not many people know about it.
(2) For Firefox browser, if the eval function is called directly, it will be executed in the caller's space; if window.eval is called, it will be executed in the global space. There are probably even fewer people who know this. After all alert(eval==window.eval) returns true!
The characteristics of Firefox's eval function are indeed very strange, but its source can also be found in the JavaScript specification:

If value of the eval property is used in any way other than a direct call (that is, other than by the explicit use of its 
name as an Identifier which is the MemberExpression in a CallExpression), or if the eval property is assigned to, 
an EvalError exception may be thrown.

It means that the execution of the eval function is and the call It is related to others, but it does not talk about its execution context. So it’s hard to say which one is right or wrong between IE and Firefox. It’s good that everyone knows the solution.

Summary:

This article shows you the use of eval in JavaScript through specific examples. I believe that friends are familiar with eval. I have further understanding and understanding of its use, I hope it will be helpful to your work!

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