When Should You Use `eval()` vs `new Function()` in JavaScript?

Are eval() and new Function() Interchangeable in JavaScript?

When working with strings containing JavaScript code, two functions commonly arise: eval() and new Function(). While both can execute code embedded in a string, they exhibit distinct characteristics and behavior.

eval() vs new Function(): The Key Differences

• Scope: The primary difference lies in their scope. eval() evaluates the code within the current execution scope, meaning it has access to local variables. On the other hand, new Function() creates a new scope and cannot access local variables directly.
• Security: eval() is notorious for security vulnerabilities. It allows the evaluation of arbitrary code, which could lead to malicious code execution if the input string is untrusted. new Function() is safer as it parses the code into a function object that runs in a separate scope.

Understanding the Code Snippet

The code snippet you provided demonstrates the different behaviors of eval() and new Function():

<code class="javascript">var evaluate = function(string) {
    return eval('(' + string + ')');
}

var func = function(string) {
    return (new Function( 'return (' + string + ')' )());
}</code>

Both functions evaluate the string "2 1" and return the result. However, if you modify the code to include a local variable "a" in the function, you will observe that eval() can access and modify the local variable, while new Function() will not.

Cautions and Best Practices

While eval() and new Function() offer convenient ways to execute dynamic code, they should be used cautiously:

• Avoid eval(): eval() is generally discouraged due to security risks and performance concerns.
• Use new Function() with care: new Function() is safer than eval(), but it can still be susceptible to injection attacks.
• Sanitize input: Always ensure that the code you are executing via eval() or new Function() is sanitized and trusted.

In most cases, it is recommended to rely on established JavaScript language features for dynamic code execution, such as template literals or the Function constructor without new.

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