In-depth understanding of the eval function in javascript_javascript skills

1) Introduce the usage of eval function in JavaScript
(2) How to execute global code within a function

► Let’s talk about the usage of eval first. The content is relatively simple, you can skip it if you are familiar with it.
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:

Copy the code The code is as follows:

var code1='"a" 2'; //Expression
var code2='{a:2}' ; //Statement
alert(eval(code1)); //->'a2'
alert(eval(code2)); //->undefined
alert(eval('(' code2 ')')); //->[object Object]

As you can see, 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. To illustrate this problem, let’s look at an example:

Copy code The code is as follows:

var s ='global'; //Define a global variable
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 experience it carefully, you can find the characteristics of the eval function. It is always executed within the context variable space (also called: package, closure) where it is called. This is true whether it is a variable definition or a function definition, so as follows The code will generate an undefined function error:

Copy the code The code is as follows:

var s= 'function test(){return 1;}'; //A function definition statement
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 not from the 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 once, or some codes are generated by Javascript itself, and we hope to use eval function to execute it.
But such dynamic code acquisition is usually done within a function, for example:

Copy code The code is as follows:

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

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

Copy code 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 define global code within a function, you can call the X2.Eval(code) method. An example is as follows:

Copy the code 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 s="local" is redefined in the demo3 function.
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, this is already provided by default Function: execScript, 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.
About the meaning That is to say, the execution of the eval function is related to the caller, but there is no issue 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.