I'm building various d3.js dashboards which frequently refer to a javascript_properties.js file which includes properties such as:
var all_charts = (function() {
return {
width:860,
height:500,
from_date:"",
to_date:"",
highlight_color:"#00FFFF"
}
}());
I use these properties frequently within various functions.
My question is:
Is there any harm in calling each property direct every time I use it or would it be more efficient to declare a local variable at the beginning of each function if a property is going to be called more than once?
To show an example. A local variable:
var width = all_charts.width;
OR calling
all_charts.width
as many times as required during a function.
There may be little discernible difference?
This isn't about memory usage, it's about lookup time.
Yes, caching the property to a local variable may make it faster when using that repeatedly afterward, as the JavaScript engine doesn't have to traverse the scope chain up to the global level to find all_charts and then look up width on it.
But, it's unlikely to make a noticeable difference unless you're using these properties hundreds of thousands of times in the same function.
Side note: There's no point to the function in the all_charts code, what you have does exactly what this does, just more indirectly:
var all_charts = {
width:860,
height:500,
from_date:"",
to_date:"",
highlight_color:"#00FFFF"
};
Related
I'm kind of curious about what the best practice is when referencing the 'global' namespace in javascript, which is merely a shortcut to the window object (or vice versia depending on how you look at it).
I want to know if:
var answer = Math.floor(value);
is better or worse than:
var answer = window.Math.floor(value);
Is one better or worse, even slightly, for performance, resource usage, or compatibility?
Does one have a slighter higher cost? (Something like an extra pointer or something)
Edit note: While I am a readability over performance nazi in most situations, in this case I am ignoring the differences in readability to focus solely on performance.
First of all, never compare things like these for performance reasons. Math.round is obviously easier on the eyes than window.Math.round, and you wouldn't see a noticeable performance increase by using one or the other. So don't obfuscate your code for very slight performance increases.
However, if you're just curious about which one is faster... I'm not sure how the global scope is looked up "under the hood", but I would guess that accessing window is just the same as accessing Math (window and Math live on the same level, as evidenced by window.window.window.Math.round working). Thus, accessing window.Math would be slower.
Also, the way variables are looked up, you would see a performance increase by doing var round = Math.round; and calling round(1.23), since all names are first looked up in the current local scope, then the scope above the current one, and so on, all the way up to the global scope. Every scope level adds a very slight overhead.
But again, don't do these optimizations unless you're sure they will make a noticeable difference. Readable, understandable code is important for it to work the way it should, now and in the future.
Here's a full profiling using Firebug:
<!DOCTYPE html>
<html>
<head>
<title>Benchmark scope lookup</title>
</head>
<body>
<script>
function bench_window_Math_round() {
for (var i = 0; i < 100000; i++) {
window.Math.round(1.23);
}
}
function bench_Math_round() {
for (var i = 0; i < 100000; i++) {
Math.round(1.23);
}
}
function bench_round() {
for (var i = 0, round = Math.round; i < 100000; i++) {
round(1.23);
}
}
console.log('Profiling will begin in 3 seconds...');
setTimeout(function () {
console.profile();
for (var i = 0; i < 10; i++) {
bench_window_Math_round();
bench_Math_round();
bench_round();
}
console.profileEnd();
}, 3000);
</script>
</body>
</html>
My results:
Time shows total for 100,000 * 10 calls, Avg/Min/Max show time for 100,000 calls.
Calls Percent Own Time Time Avg Min Max
bench_window_Math_round
10 86.36% 1114.73ms 1114.73ms 111.473ms 110.827ms 114.018ms
bench_Math_round
10 8.21% 106.04ms 106.04ms 10.604ms 10.252ms 13.446ms
bench_round
10 5.43% 70.08ms 70.08ms 7.008ms 6.884ms 7.092ms
As you can see, window.Math is a really bad idea. I guess accessing the global window object adds additional overhead. However, the difference between accessing the Math object from the global scope, and just accessing a local variable with a reference to the Math.round function isn't very great... Keep in mind that this is 100,000 calls, and the difference is only 3.6ms. Even with one million calls you'd only see a 36ms difference.
Things to think about with the above profiling code:
The functions are actually looked up from another scope, which adds overhead (barely noticable though, I tried importing the functions into the anonymous function).
The actual Math.round function adds overhead (I'm guessing about 6ms in 100,000 calls).
This can be an interest question if you want to know how the Scope Chain and the Identifier Resolution process works.
The scope chain is a list of objects that are searched when evaluating an identifier, those objects are not accessible by code, only its properties (identifiers) can be accessed.
At first, in global code, the scope chain is created and initialised to contain only the global object.
The subsequent objects in the chain are created when you enter in function execution context and by the with statement and catch clause, both also introduce objects into the chain.
For example:
// global code
var var1 = 1, var2 = 2;
(function () { // one
var var3 = 3;
(function () { // two
var var4 = 4;
with ({var5: 5}) { // three
alert(var1);
}
})();
})();
In the above code, the scope chain will contain different objects in different levels, for example, at the lowest level, within the with statement, if you use the var1 or var2 variables, the scope chain will contain 4 objects that will be needed to inspect in order to get that identifier: the one introduced by the with statement, the two functions, and finally the global object.
You also need to know that window is just a property that exists in the global object and it points to the global object itself. window is introduced by browsers, and in other environments often it isn't available.
In conclusion, when you use window, since it is just an identifier (is not a reserved word or anything like that) and it needs to pass all the resolution process in order to get the global object, window.Math needs an additional step that is made by the dot (.) property accessor.
JS performance differs widely from browser to browser.
My advice: benchmark it. Just put it in a for loop, let it run a few million times, and time it.... see what you get. Be sure to share your results!
(As you've said) Math.floor will probably just be a shortcut for window.Math (as window is a Javascript global object) in most Javascript implementations such as V8.
Spidermonkey and V8 will be so heavily optimised for common usage that it shouldn't be a concern.
For readability my preference would be to use Math.floor, the difference in speed will be so insignificant it's not worth worrying about ever. If you're doing a 100,000 floors it's probably time to switch that logic out of the client.
You may want to have a nose around the v8 source there's some interesting comments there about shaving nanoseconds off functions such as this int.Parse() one.
// Some people use parseInt instead of Math.floor. This
// optimization makes parseInt on a Smi 12 times faster (60ns
// vs 800ns). The following optimization makes parseInt on a
// non-Smi number 9 times faster (230ns vs 2070ns). Together
// they make parseInt on a string 1.4% slower (274ns vs 270ns).
As far as I understand JavaScript logic, everything you refer to as something is searched in the global variable scope. In browser implementations, the window object is the global object. Hence, when you are asking for window.Math you actually have to de-reference what window means, then get its properties and find Math there. If you simply ask for Math, the first place where it is sought, is the global object.
So, yes- calling Math.something will be faster than window.Math.something.
D. Crockeford talks about it in his lecture http://video.yahoo.com/watch/111593/1710507, as far as I recall, it's in the 3rd part of the video.
If Math.round() is being called in a local/function scope the interpreter is going to have to check first for a local var then in the global/window space. So in local scope my guess would be that window.Math.round() would be very slightly faster. This isn't assembly, or C or C++, so I wouldn't worry about which is faster for performance reasons, but if out of curiosity, sure, benchmark it.
I'm looking for the standard way to calculate a variable once, then access it within the scope of every execution of a function, without relying on global variables.
This seems like a standard use of prototype properties (variables) - but every example I can find on JS prototypes is based on prototype methods (functions). The only thing I can find about setting properties / variables in a prototype is a question from someone who also couldn't find any information about these, asking if it's good or bad practice (tldr: it's fine, but remember it's rarely worth sacrificing readability for tiny performance gains).
I've got a way to set and get prototype properties that works, but feels clunky as it depends on a reference to the function (essentially var prop = thisfunctionname.prototype.someprop). Since I found it through trial and error, I'd like to ask if there's a cleaner, more standard way to get these prototype properties from within the function, without going back up to the scope around the function and getting the function from there?
Here's a simplified light-hearted example: an imaginary jQuery plugin that adds a number to another number then returns it in a sentence with the user's name. We want to ask the user their name only once, then store that name for re-use within scope:
(function($) {
var sum = function( num1,num2 ) {
var result = num1 + num2;
// This works, but seems clunky since it depends on the variable `sum`
// from the scope around this function - is there a better way?
var name = sum.prototype.name;
$(this).text( num1+' plus '+num2+' is '+result+', '+name+'.');
return $(this);
};
var name = prompt('Please enter your name','');
// Is there a better way to set this default variable to be accessible
// in all calls to this function?
sum.prototype.name = name;
$.fn.basicArithmetic = sum;
})(jQuery);
// end of plugin. Example usage...
$('<p/>').basicArithmetic(1,5).appendTo('body');
$('<p/>').basicArithmetic(2,2).appendTo('body');
$('<p/>').basicArithmetic(25,30).appendTo('body');
$('<p/>').basicArithmetic(92.3,15.17).appendTo('body');
Live jsbin example. More realistic real-life use cases would be when the calculation for the property is expensive in memory usage, or destructive (e.g. requires changing the DOM during calculation).
Two different answers, really:
The usual way is to use a variable within a scoping function (you already have one handy in your example); no prototypes involved at all.
(function($) {
var name;
name = prompt('Please enter your name','');
function sum( num1,num2 ) {
var result = num1 + num2;
$(this).text( num1+' plus '+num2+' is '+result+', '+name+'.');
return $(this);
}
$.fn.basicArithmetic = sum;
})(jQuery);
Updated JSBin Example | Source
(Side note: I also changed your anonymous function expression into a named function declaration, but it doesn't really matter in this case.)
The usual way in a jQuery plug-in is to store the data on the element(s) the plug-in is being applied to. That doesn't work for the example you gave, which requires that the data be global to the plug-in, but normally (not always, just normally) plug-ins keep only instance-specific information, which you'd normally store on elements (probably via the data function).
This might sound like a noob question but here goes;
Basically, I'm passing a large amount of data from one object to another. Below is a simplified example.
// Example 1
function Person(hugeData) {
this.info = function() {
console.log(hugeData);
}
}
Homer = new Person(hugeData);
Homer.info();
Compared with
// Example 2
function Person() {
var hugeData;
this.set = function(data) {
hugeData = data;
}
this.info = function() {
console.log(hugeData);
}
}
Homer = new Person();
Homer.set(hugeData);
Homer.info();
Is there much of a difference performance-wise between the two code snippets? Please focus on the context of the example rather than the code itself (setting object variable vs passing by arguments).
While the example above is for Javascript, I would also like to know if the same principle applies for other programming languages like PHP.
Thanks.
No, not at all.
Without going into much detail now, both, formal paramters and local variables are stored within the such called Activation Object (in ES3) or the Lexical Environment Record (ES5) under the hood.
So access times should be identical by spec.
If you want to know the details, checkout:
http://dmitrysoshnikov.com/ecmascript/javascript-the-core/
and
http://dmitrysoshnikov.com/ecmascript/es5-chapter-3-2-lexical-environments-ecmascript-implementation/
Testcase: http://jsperf.com/formal-parameters-vs-local-variables-access-time
I assume the major point of your question is whether this line...
hugeData = data;
... in your code may affect the performance.
And the answer is no, it's not (at least not so it can affect the application's performance).
If hugeData refers to an object (and remember arrays in JS are essentially objects), it actually stores only a reference to this object. And the reference is what will be copied, without any duplication of the object's contents.
If hugeData refers to a string, it can be a bit more complicated... but as far as I know, most modern browsers (check MDN, as example) now implement 'copy-on-writing' technique. In other words, the string won't be duplicated here as well.
Passing by variable is the best thing. Because it's obvious your huge data is not a primitive so you are going to only refer that in the functions context.
Look at jAndy's answer.
It's hard to generalize in this case because every implementation of Javascript is different.
I could imagine that when you pass a big hunk of data with the creation of an object, you are saving a reallocation of memory, which would happen when you create the object with few data first and then add a lot of data to it later. But most JS implementation will store the big data chunk in a reference anyway, so it is unlikely that it really matters.
Within instances of objects I like to use the closure mechanism to simulate private member variables. For a potential large number of created objects I don't need some of the private members though, but I have to declare them for them to be used in the closure, like "one", "two" and "three" here:
var obj=function()
{
var one;
var two;
var three;
var M=function()
{
one=5;
};
};
(Don't mind that this is not actually a working example of my setup, it's just to demonstrate the use of the closure over the three vars with M.)
Do the var statements themselves already consume memory, or does that depend on actually assigning something to these vars like with "one"?
The interpreter has to store information about scope - one = 5 will change the local variable one instead of creating a global variable (which would happen with e.g. four = 5). This information must cost some memory somehow. This memory usage also applies before assigning a value to one, because the information has to be available at the time you're assigning.
How much memory it will cost is difficult to say since it differs per interpreter. I guess it isn't enough to worry about.
Note that two/three are not used at all and may be garbage collected in this example. (Actually, you don't expose M either, so everything may be garbage collected right away in this example.)
When declaring a variable without assigning a value to it, there still needs to be some memory available for it, otherwise you cannot make a reference to the variable later in the program. I don't think it's a noticeable amount of memory being used and won't make a difference.
When declaring a variable, a memory space is reserved for it and allow you to store to or retrieve from that memory with the name you chose for the 3 variables. Such a space is empty until you fill it with a value (two/tree will remain null). This is performed with the assignment operation. The assignment operation gives a value to a variable.
I'm kind of curious about what the best practice is when referencing the 'global' namespace in javascript, which is merely a shortcut to the window object (or vice versia depending on how you look at it).
I want to know if:
var answer = Math.floor(value);
is better or worse than:
var answer = window.Math.floor(value);
Is one better or worse, even slightly, for performance, resource usage, or compatibility?
Does one have a slighter higher cost? (Something like an extra pointer or something)
Edit note: While I am a readability over performance nazi in most situations, in this case I am ignoring the differences in readability to focus solely on performance.
First of all, never compare things like these for performance reasons. Math.round is obviously easier on the eyes than window.Math.round, and you wouldn't see a noticeable performance increase by using one or the other. So don't obfuscate your code for very slight performance increases.
However, if you're just curious about which one is faster... I'm not sure how the global scope is looked up "under the hood", but I would guess that accessing window is just the same as accessing Math (window and Math live on the same level, as evidenced by window.window.window.Math.round working). Thus, accessing window.Math would be slower.
Also, the way variables are looked up, you would see a performance increase by doing var round = Math.round; and calling round(1.23), since all names are first looked up in the current local scope, then the scope above the current one, and so on, all the way up to the global scope. Every scope level adds a very slight overhead.
But again, don't do these optimizations unless you're sure they will make a noticeable difference. Readable, understandable code is important for it to work the way it should, now and in the future.
Here's a full profiling using Firebug:
<!DOCTYPE html>
<html>
<head>
<title>Benchmark scope lookup</title>
</head>
<body>
<script>
function bench_window_Math_round() {
for (var i = 0; i < 100000; i++) {
window.Math.round(1.23);
}
}
function bench_Math_round() {
for (var i = 0; i < 100000; i++) {
Math.round(1.23);
}
}
function bench_round() {
for (var i = 0, round = Math.round; i < 100000; i++) {
round(1.23);
}
}
console.log('Profiling will begin in 3 seconds...');
setTimeout(function () {
console.profile();
for (var i = 0; i < 10; i++) {
bench_window_Math_round();
bench_Math_round();
bench_round();
}
console.profileEnd();
}, 3000);
</script>
</body>
</html>
My results:
Time shows total for 100,000 * 10 calls, Avg/Min/Max show time for 100,000 calls.
Calls Percent Own Time Time Avg Min Max
bench_window_Math_round
10 86.36% 1114.73ms 1114.73ms 111.473ms 110.827ms 114.018ms
bench_Math_round
10 8.21% 106.04ms 106.04ms 10.604ms 10.252ms 13.446ms
bench_round
10 5.43% 70.08ms 70.08ms 7.008ms 6.884ms 7.092ms
As you can see, window.Math is a really bad idea. I guess accessing the global window object adds additional overhead. However, the difference between accessing the Math object from the global scope, and just accessing a local variable with a reference to the Math.round function isn't very great... Keep in mind that this is 100,000 calls, and the difference is only 3.6ms. Even with one million calls you'd only see a 36ms difference.
Things to think about with the above profiling code:
The functions are actually looked up from another scope, which adds overhead (barely noticable though, I tried importing the functions into the anonymous function).
The actual Math.round function adds overhead (I'm guessing about 6ms in 100,000 calls).
This can be an interest question if you want to know how the Scope Chain and the Identifier Resolution process works.
The scope chain is a list of objects that are searched when evaluating an identifier, those objects are not accessible by code, only its properties (identifiers) can be accessed.
At first, in global code, the scope chain is created and initialised to contain only the global object.
The subsequent objects in the chain are created when you enter in function execution context and by the with statement and catch clause, both also introduce objects into the chain.
For example:
// global code
var var1 = 1, var2 = 2;
(function () { // one
var var3 = 3;
(function () { // two
var var4 = 4;
with ({var5: 5}) { // three
alert(var1);
}
})();
})();
In the above code, the scope chain will contain different objects in different levels, for example, at the lowest level, within the with statement, if you use the var1 or var2 variables, the scope chain will contain 4 objects that will be needed to inspect in order to get that identifier: the one introduced by the with statement, the two functions, and finally the global object.
You also need to know that window is just a property that exists in the global object and it points to the global object itself. window is introduced by browsers, and in other environments often it isn't available.
In conclusion, when you use window, since it is just an identifier (is not a reserved word or anything like that) and it needs to pass all the resolution process in order to get the global object, window.Math needs an additional step that is made by the dot (.) property accessor.
JS performance differs widely from browser to browser.
My advice: benchmark it. Just put it in a for loop, let it run a few million times, and time it.... see what you get. Be sure to share your results!
(As you've said) Math.floor will probably just be a shortcut for window.Math (as window is a Javascript global object) in most Javascript implementations such as V8.
Spidermonkey and V8 will be so heavily optimised for common usage that it shouldn't be a concern.
For readability my preference would be to use Math.floor, the difference in speed will be so insignificant it's not worth worrying about ever. If you're doing a 100,000 floors it's probably time to switch that logic out of the client.
You may want to have a nose around the v8 source there's some interesting comments there about shaving nanoseconds off functions such as this int.Parse() one.
// Some people use parseInt instead of Math.floor. This
// optimization makes parseInt on a Smi 12 times faster (60ns
// vs 800ns). The following optimization makes parseInt on a
// non-Smi number 9 times faster (230ns vs 2070ns). Together
// they make parseInt on a string 1.4% slower (274ns vs 270ns).
As far as I understand JavaScript logic, everything you refer to as something is searched in the global variable scope. In browser implementations, the window object is the global object. Hence, when you are asking for window.Math you actually have to de-reference what window means, then get its properties and find Math there. If you simply ask for Math, the first place where it is sought, is the global object.
So, yes- calling Math.something will be faster than window.Math.something.
D. Crockeford talks about it in his lecture http://video.yahoo.com/watch/111593/1710507, as far as I recall, it's in the 3rd part of the video.
If Math.round() is being called in a local/function scope the interpreter is going to have to check first for a local var then in the global/window space. So in local scope my guess would be that window.Math.round() would be very slightly faster. This isn't assembly, or C or C++, so I wouldn't worry about which is faster for performance reasons, but if out of curiosity, sure, benchmark it.