Introduction to DSL meta programming in JavaScript _javascript tips

When looking at JavaScript template source code, found that there is a very interesting use to generate functions, think this is not metaprogramming it?

JavaScript Meta Programming

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Metaprogramming (metaprogramming) refers to the writing of a class of computer programs that write or manipulate other programs (or themselves) as their data, or work at runtime to complete portions that should have been done at compile time.

JavaScript Eval

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The eval () function computes a string and executes the JavaScript code in it.

You can use the following:

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Eval ("X=10;y=20;document.write (x*y)")

Of course, this is only used to carry out a function, but the goods cost-effective, but also easy to make mistakes.

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The Eval function should be avoided as much as possible.

So the better way is to use the new Function ()

A big difference between using the new function () and eval () is that Eval is more than just a function,

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Eval () evaluates a string as a JavaScript expression in the current execution scope and can access local variables. The New function () resolves the JavaScript code that is stored in a string that converts to a function object and then can be invoked. Because the code runs in a separate scope, local variables cannot be accessed.

This means that eval () interferes with the scope of the current function.

JavaScript New Function ()

function constructor to create a new function object. In JavaScript, each function is actually an object of functions. The function objects that are generated by using the functions constructor are parsed when they are created. This is less efficient than using a function declaration and calling it in your code, because functions declared using function statements are parsed with other statements.

The New Function () is relatively inefficient in comparison, and this is what we can predict in the current situation.

A simple example is shown below:

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var add = new Function ([' X ', ' y '], ' return x + y ');

The new function () resolves the string to a function ... And then we can do it through apply.

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Function.apply (null, args)

And that's what I see in the JavaScript Template:

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New Function (
Tmpl.arg + ', Tmpl ',
"var _e=tmpl.encode" + Tmpl.helper + ", _s= '" +
Str.replace (Tmpl.regexp, Tmpl.func) +
"'; return _s;"
);

Of course we have other ways.