eval is not a method in es6; this method is in JavaScript and is used to calculate JavaScript strings and execute the string as script code. If the incoming string is an expression, the expression is returned. The evaluation result, otherwise undefined is returned, and the syntax is "eval(string)".
The operating environment of this tutorial: Windows 10 system, ECMAScript version 6.0, Dell G3 computer.
eval is not a method in es6
The eval() function calculates a JavaScript string and executes it as script code.
If the parameter is an expression, the eval() function will execute the expression. If the argument is a Javascript statement, eval() will execute the Javascript statement.
eval() function will execute the incoming string as JavaScript code. If the incoming string is an expression, it will return the expression evaluation result, otherwise it will return undefined.
Description
If the parameter passed into eval() is not a string, the parameter will be returned directly.
When eval() is called directly in non-strict mode, variables declared using var and functions declared using function will modify the current lexical scope, and variables declared using let and const will be modified. The current lexical scope will not be modified, but a new lexical scope will be created within the current lexical scope.
When eval() is indirectly referenced in non-strict mode, it will run directly in the global environment. Variables declared using var and functions declared using function will modify the global lexical scope. Variables declared using let and const will not modify the global lexical scope, but will create a new lexical scope in the global environment.
When eval() is called directly in strict mode, a new independent lexical scope will be created at the current time.
When eval() is indirectly referenced in strict mode (only when strict mode is turned on in the string within eval(), the string will be executed in strict mode), it will be executed in strict mode. The global environment creates a new independent lexical scope.
Using window.eval() is equivalent to indirect reference to eval().
#The code executed in eval() can only call the JS interpreter (Interpreter) to interpret and execute, and cannot be optimized by the just-in-time compiler (JIT Compiler). The code executed in eval() The code may cause the JS engine to perform variable lookup and assignment in the generated machine code, causing performance problems.
#Improper use of eval() may cause the string executed inside to be vulnerable to malicious modification, causing security issues (such as XSS attacks).
Using eval() will interfere with the behavior of code minification tools. Code compression tools generally rename local variable names to shorter variable names (such as a and b, etc.) to reduce code size. When eval() is used, since external local variables may be accessed by eval(), the code compression tool will not compress the local variable names that may be accessed by eval(), which will reduce the code compression rate.
Syntax
eval(string)
The example is as follows:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>123</title>
</head>
<body>
<script>
eval("x=10;y=20;document.write(x*y)");
document.write("<br>" + eval("2+2"));
document.write("<br>" + eval(x+17));
</script>
</body>
</html>Output result:
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