Geoff Geoff - 3 months ago 12
Node.js Question

Extending Array.prototype in Node.js, from a require'd file

I have the following saved in test.js. It successfully extends Array in the browser, but it doesn't seem to work with node and require. Can someone explain what's wrong here?

(function() {

Array.prototype.max = function() {
return console.log("Array.prototype.max");
};
return Array.max = function() {
return console.log("Array.max");
};

}).call(this);


Then, from a terminal:

> My-MacBook-Pro: me$ node
> var test = require("./test")
> [1,2,3].max()
TypeError: Object 1,2,3 has no method 'max'
at [object Context]:1:9
at Interface.<anonymous> (repl.js:171:22)
at Interface.emit (events.js:64:17)
at Interface._onLine (readline.js:153:10)
at Interface._line (readline.js:408:8)
at Interface._ttyWrite (readline.js:585:14)
at ReadStream.<anonymous> (readline.js:73:12)
at ReadStream.emit (events.js:81:20)
at ReadStream._emitKey (tty_posix.js:307:10)
at ReadStream.onData (tty_posix.js:70:12)
> Array.max()
TypeError: Object function Array() { [native code] } has no method 'max'
at [object Context]:1:7
at Interface.<anonymous> (repl.js:171:22)
at Interface.emit (events.js:64:17)
at Interface._onLine (readline.js:153:10)
at Interface._line (readline.js:408:8)
at Interface._ttyWrite (readline.js:585:14)
at ReadStream.<anonymous> (readline.js:73:12)
at ReadStream.emit (events.js:81:20)
at ReadStream._emitKey (tty_posix.js:307:10)
at ReadStream.onData (tty_posix.js:70:12)

Answer

Every command in the REPL is executed via vm.runInContext with a shared context object. This object is created at REPL initialization by copying everything from the global object. Since the require'd module will only extend Array.prototype after it has been copied to the context object, the modified version is never exposed.

Or at least that's what I could deduce from the source code. I know nothing about the inner workings of V8 :) And as you probably have found out by now, your example works fine outside the REPL.

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