When to Use `eval()` vs. `new Function()` in JavaScript: What Are the Key Differences?

Exploring the Differences Between eval() and new Function()

In JavaScript, eval() and new Function() both allow for dynamic code execution. However, they differ significantly in their underlying mechanisms and scope.

eval()

eval() evaluates a string as a JavaScript expression within the current execution scope. This means it has access to local variables within the scope where it is called.

new Function()

new Function() parses a JavaScript code string into a function object. This function is then called and its code is executed in a separate scope. It does not have access to local variables outside of this scope.

Practical Differences

Consider the following example:

var evaluate = function(string) {
    return eval('(' + string + ')');
}

var func = function(string) {
    return (new Function( 'return (' + string + ')' )());
}

console.log(evaluate('2 + 1'));
console.log(func('2 + 1'));

Both evaluate() and func() will return the result of the arithmetic expression '2 1'. However, if the following code were to be executed within a function:

var a = 11;
evaluate('a = 22');

The value of a would be changed to 22 because eval() uses the current execution scope. In contrast, if func() were used instead, the value of a would remain unchanged because it operates in a separate scope.

Usage Considerations

While eval() and new Function() can be useful in certain scenarios, they should be used with caution due to security risks and potential for unintended consequences. Evaling untrusted data can expose vulnerabilities, and both functions can create global objects that can pollute the global scope.

In general, it is recommended to avoid using eval() and new Function() unless absolutely necessary. Instead, consider safer alternatives such as custom functions or using the eval() alternate syntax (eval("1 1")).

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