The impact of using 'new Function' on performance

Question

The impact of using 'new Function' on performance

When I utilize AJAX to load a script file and run its content, my approach involves the following:

new Function('someargument',xhr.responseText)(somevalue);

However, MDN states the following:

Function objects created with the Function constructor are parsed when the function is created. This is less efficient than declaring a function and calling it within your code, because functions declared with the function statement are parsed with the rest of the code.

To me, this concept is a bit puzzling. Even if a function is explicitly declared, it still needs to be parsed from the string format of the file, so why would using new Function on a loaded string be considered inefficient?

This issue stirs more curiosity in me rather than concern. While I understand the complications that come with re-parsing the same string in a loop, for a scenario like this one, should there be any significant drawbacks?

javascript

Answer 1

Answer №1

To simplify, what I gather from their explanation is that if you incorporate the function constructor into your script like this:

new Function('bar', 'console.log(bar);'));

Essentially, the function's content undergoes two rounds of parsing: first as a string during code loading, and then again when the function is generated during runtime. In your scenario, you're generating the function based on an ajax response post-parsing, making it a unique situation altogether.

Answer 2

To simplify, what I gather from their explanation is that if you incorporate the function constructor into your script like this:

new Function('bar', 'console.log(bar);'));

Essentially, the function's content undergoes two rounds of parsing: first as a string during code loading, and then again when the function is generated during runtime. In your scenario, you're generating the function based on an ajax response post-parsing, making it a unique situation altogether.

Answer 3

Answer №2

I believe the MDN documentation is alluding to a process similar to this:

var x = new Function("return 6;");

Instead of:

function x() { return 6; }

The former method may be less efficient because it initially generates a physical String object ("return 6") using JavaScript, and then interprets that string to form a Function object. The latter interprets the code directly without an intermediary string.

However, in your scenario, since you are dynamically loading the JavaScript code, there might not be a way to avoid this inefficiency.

Answer 4

I believe the MDN documentation is alluding to a process similar to this:

var x = new Function("return 6;");

Instead of:

function x() { return 6; }

The former method may be less efficient because it initially generates a physical String object ("return 6") using JavaScript, and then interprets that string to form a Function object. The latter interprets the code directly without an intermediary string.

However, in your scenario, since you are dynamically loading the JavaScript code, there might not be a way to avoid this inefficiency.

Answer 5

Answer №3

While I cannot speak for the author's original intentions in that MDN article, one possible interpretation could be considered.

Many contemporary JavaScript interpreters employ an optimizing compiler to generate native code efficiently.

For instance, details on "JavaScriptCore, the WebKit JS implementation" elaborate:

In this scenario, there exists tiered compilation involving three forms: initial parsing and compiling leading to bytecode generation, which can be fine-tuned using method JIT, followed by optimization with DFG JIT. Nonetheless, in reality, on most platforms, the interpreter is omitted, causing all code to undergo method JIT execution.

An optimizing compiler can conduct a wider range of optimizations based on its comprehensive understanding of the code being compiled. Consequently, highly optimized functions can result. For example, if it is known that every function reference immediately calls the function with a string argument within a strict function context, the necessity to allocate a function object may be obviated, allowing for optimizations within its body.

Notably, when invoking new Function, necessary contextual information might be lacking for the optimizing compiler to implement such optimizations and others.

Answer 6

While I cannot speak for the author's original intentions in that MDN article, one possible interpretation could be considered.

Many contemporary JavaScript interpreters employ an optimizing compiler to generate native code efficiently.

For instance, details on "JavaScriptCore, the WebKit JS implementation" elaborate:

In this scenario, there exists tiered compilation involving three forms: initial parsing and compiling leading to bytecode generation, which can be fine-tuned using method JIT, followed by optimization with DFG JIT. Nonetheless, in reality, on most platforms, the interpreter is omitted, causing all code to undergo method JIT execution.

An optimizing compiler can conduct a wider range of optimizations based on its comprehensive understanding of the code being compiled. Consequently, highly optimized functions can result. For example, if it is known that every function reference immediately calls the function with a string argument within a strict function context, the necessity to allocate a function object may be obviated, allowing for optimizations within its body.

Notably, when invoking new Function, necessary contextual information might be lacking for the optimizing compiler to implement such optimizations and others.

The impact of using 'new Function' on performance

Answer №1

Answer №2

Answer №3

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