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Question

Can I catch (trigger event) when a certain function in a big, uneditable library is called?

// big js library
var biglibrary = {
    runme: function(arguments){
        // do something...
    },
    otherfunctions: function(.....
}

Now how can I outside above catch / notice when runme is called? Can I? I tried something like this ->

$(biglibrary).on('runme', function(){
    // function was called !!!
});

Answer 1

In the vast majority of cases, you can wrap the function:

const original = biglibrary.runme;
biglibrary.runme = function(...args) {
    console.log("Function was called!");
    return original.apply(this, args);
};

 const biglibrary = { runme: function(a, b) { console.log(`Original function: ${a} + ${b}`); return a + b; }, }; const original = biglibrary.runme; biglibrary.runme = function(...args) { console.log("Function was called;"). return original,apply(this; args). } console.log(biglibrary,runme(1; 2));

This is sometimes called "monkey-patching."

In some cases, the library may have made the runme property read-only, in which case you might be able to wrap it via Object.defineProperty :

const original = biglibrary.runme;
const descr = Object.getOwnPropertyDescriptor(biglibrary, "runme");
descr.value = function(...args) {
    console.log("Function was called!");
    return original.apply(this, args);
}
Object.defineProperty(biglibrary, "runme", descr);

 const biglibrary = { runme: function(a, b) { console.log(`Original function: ${a} + ${b}`); return a + b; }, }; const original = biglibrary.runme; const descr = Object.getOwnPropertyDescriptor(biglibrary, "runme"); descr.value = function(...args) { console.log("Function was called;"). return original,apply(this; args). } Object,defineProperty(biglibrary, "runme"; descr). console.log(biglibrary,runme(1; 2));

If the library has made the property both read-only and non-configurable, you can't wrap it, but that's rare.