Extending Number.prototype in javascript and the M

问题:

I've always wondered why Javascript has the global Math object instead of giving numbers their own methods. Is there a good reason for it?

Also are there any drawbacks (other than efficiency) to doing something like this?:

Number.prototype.round = function(){
    return Math.round(this);
};

Just to make clear, I understand that constants like π need somewhere and functions that are applied on more than one number like min/max. The question was mainly concerning methods which only effect a single number such as round, abs, sin, pow, etc.

回答1:

The reason for the Math object is simple: "because Java does it". Not the best of reasons, but here we are. I guess things made more sense back then, before Douglas Crockford started his campaign to suppress half the language*. Originally you were "allowed", or meant, to do things like this:

with (Math) {
  var n = min( round(a) * round(b), sqrt(c) );
  var result = exp( n + d );
}

The drawback to extending Number.prototype is that someone else might do the same thing. Or worse, for example, define Number.prototype.round as a symmetrical rounding function.

If you are looking for ways to make your life easier, why stop there? Why not simply include Math functions as global functions?

var m = 'abs acos asin atan atan2 ceil cos exp floor log max min ' +
        'pow random round sin sqrt tan PI').split(' ');
for (var i=0,l=m.length; i<l; i++) {
  window[ m[i] ] = Math[ m[i] ];
}

This will drop all the math functions into the global scope, effectively allowing you to stop typing "Math." Ask yourself: Is there any real difference between extending Number and extending window with these functions?

* Before you flame me: The Crockford comment is not meant to be taken too seriously. I do agree with him that with is very dangerous in an implicit global environment.

回答2:

There is no drawback in extending Number.prototype other than confusing other people. What's the point? What is better in using value.round() instead of Math.round(value)?

There are several good reasons for the Math object:

1. It works for non-numbers, too: Math.round("5") works whereas value.round() won't work when value is a string (for example, the value of a textbox)
2. Some members of the Math object don't belong to a "primary" number value, like Math.min() or Math.max(). Or do you want to use it like a.max(b)?
3. Other members are global and do not belong to a specialized number. Examples are constants like Math.PI or the function Math.random().

回答3:

Try doing 123.round();

Your javascript console will throw a few hundred of errors to your eyes :P, nah ...

You can do:

Number.prototype.x then: (123).x(); but never 123.x();

回答4:

I think Math is more than set of Number methods. It's usage wider. Say, using NumberVariable.PI can be confusing. Same with random numbers generation.

Also I think extending number is OK, because it's the part of JS nature. Maybe someone will correct me if I am wrong here.

回答5:

I believe calling it this way works as well, since Number's prototype functions work just like every other Object's prototype functions do:

5.5["round"]();
//should return 6

回答6:

So, the conversation on whether its a good idea or not aside, you can do this fine.

You can do 123.x() but the interpreter for js is broken (as it doesn't interpret the dot as a message dispatch)

Weirdly, you can use 123 .x() (with a space between the number and the dot) instead, and it'll work.

123..also works but that's because you've effectively made a decimal and then dispatching to it

(typeof 123.) === 'number') // true

Try:

Number.prototype.times = function(other) { return this * other }; 3 .times(5);

in the console.

回答7:

My view on this is that if you do the proper checks as to not overwrite native functionality, name with understanding of native naming standards and it makes your code more readable and manageable, then make your code comfortable and convenient.

if (Number.prototype.round == null)
    Number.prototype.round = function() { return Math.round(this); }

AS for using this, because of the nature of javascript, I believe the best way to do this is by:

• Wrapping the number in a variable instead of a simple static value (Most often the case)
  
  • nNum.round();
• Wrapping the simple static value in parentheses :
  • (123).round();
• Calling using square bracket notation
  • 123["round"].call();

回答8:

You can do this

Number.prototype.round = function() {
    return Math.round(this.valueOf());
};

https://www.w3schools.com/jsref/jsref_prototype_num.asp