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I wanted to extend String object prototype with some utility method. It worked, but the performance was surprisingly low. Passing a string to a function is 10x times faster than overriding the String.prototype method that is doing the same thing. To make sure this really happens I created a very simple count() function and the corresponding methods.

(I was experimenting, and created three different versions of the method.)

function count(str, char) {
    var n = 0;
    for (var i = 0; i < str.length; i++) if (str[i] == char) n++;
    return n;
}

String.prototype.count = function (char) {
    var n = 0;
    for (var i = 0; i < this.length; i++) if (this[i] == char) n++;
    return n;
}

String.prototype.count_reuse = function (char) {
    return count(this, char)
}

String.prototype.count_var = function (char) {
    var str = this;
    var n = 0;
    for (var i = 0; i < str.length; i++) if (str[i] == char) n++;
    return n;
}

// Here is how I measued speed, using Node.js 6.1.0

var STR ='0110101110010110100111010011101010101111110001010110010101011101101010101010111111000';
var REP = 1e3//6;

console.time('func')
for (var i = 0; i < REP; i++) count(STR,'1')
console.timeEnd('func')

console.time('proto')
for (var i = 0; i < REP; i++) STR.count('1')
console.timeEnd('proto')

console.time('proto-reuse')
for (var i = 0; i < REP; i++) STR.count_reuse('1')
console.timeEnd('proto-reuse')

console.time('proto-var')
for (var i = 0; i < REP; i++) STR.count_var('1')
console.timeEnd('proto-var')

Results:

func: 705 ms
proto: 10011 ms
proto-reuse: 10366 ms
proto-var: 9703 ms

As you can see the difference is dramatic.

The below proves that performance of method calls is neglectably slower, and that the function code it self is slower for methods.

function count_dummy(str, char) {
    return 1234;
}

String.prototype.count_dummy = function (char) {
    return 1234; // Just to prove that accessing the method is not the bottle-neck.
}

console.time('func-dummy')
for (var i = 0; i < REP; i++) count_dummy(STR,'1')
console.timeEnd('func-dummy')

console.time('proto-dummy')
for (var i = 0; i < REP; i++) STR.count_dummy('1')
console.timeEnd('proto-dummy')

console.time('func-dummy')
for (var i = 0; i < REP; i++) count_dummy(STR,'1')
console.timeEnd('func-dummy')

Results:

func-dummy: 0.165ms
proto-dummy: 0.247ms

Although on huge repetitions (like 1e8) prototyped methods proves to be 10x times slower than functions, this can be ignored for this case.

All this may be related only to a String object, because simple generic objects perform about the same when you pass them to functions or call their methods:

var A = { count: 1234 };

function getCount(obj) { return obj.count }

A.getCount = function() { return this.count }

console.time('func')
for (var i = 0; i < 1e9; i++) getCount(A)
console.timeEnd('func')

console.time('method')
for (var i = 0; i < 1e9; i++) A.getCount()
console.timeEnd('method')

Results:

func: 1689.942ms
method: 1674.639ms

I've been searching on Stackoverflow and binging, but other that the remendation "do not extend String or Array because you pollute the name space" (which is not a problem for my particular project), I cannot find anything related to performance of methods pared to functions. So should I simply forget about extending the String object due to performance drop of added methods or there is more about it?

I wanted to extend String object prototype with some utility method. It worked, but the performance was surprisingly low. Passing a string to a function is 10x times faster than overriding the String.prototype method that is doing the same thing. To make sure this really happens I created a very simple count() function and the corresponding methods.

(I was experimenting, and created three different versions of the method.)

function count(str, char) {
    var n = 0;
    for (var i = 0; i < str.length; i++) if (str[i] == char) n++;
    return n;
}

String.prototype.count = function (char) {
    var n = 0;
    for (var i = 0; i < this.length; i++) if (this[i] == char) n++;
    return n;
}

String.prototype.count_reuse = function (char) {
    return count(this, char)
}

String.prototype.count_var = function (char) {
    var str = this;
    var n = 0;
    for (var i = 0; i < str.length; i++) if (str[i] == char) n++;
    return n;
}

// Here is how I measued speed, using Node.js 6.1.0

var STR ='0110101110010110100111010011101010101111110001010110010101011101101010101010111111000';
var REP = 1e3//6;

console.time('func')
for (var i = 0; i < REP; i++) count(STR,'1')
console.timeEnd('func')

console.time('proto')
for (var i = 0; i < REP; i++) STR.count('1')
console.timeEnd('proto')

console.time('proto-reuse')
for (var i = 0; i < REP; i++) STR.count_reuse('1')
console.timeEnd('proto-reuse')

console.time('proto-var')
for (var i = 0; i < REP; i++) STR.count_var('1')
console.timeEnd('proto-var')

Results:

func: 705 ms
proto: 10011 ms
proto-reuse: 10366 ms
proto-var: 9703 ms

As you can see the difference is dramatic.

The below proves that performance of method calls is neglectably slower, and that the function code it self is slower for methods.

function count_dummy(str, char) {
    return 1234;
}

String.prototype.count_dummy = function (char) {
    return 1234; // Just to prove that accessing the method is not the bottle-neck.
}

console.time('func-dummy')
for (var i = 0; i < REP; i++) count_dummy(STR,'1')
console.timeEnd('func-dummy')

console.time('proto-dummy')
for (var i = 0; i < REP; i++) STR.count_dummy('1')
console.timeEnd('proto-dummy')

console.time('func-dummy')
for (var i = 0; i < REP; i++) count_dummy(STR,'1')
console.timeEnd('func-dummy')

Results:

func-dummy: 0.165ms
proto-dummy: 0.247ms

Although on huge repetitions (like 1e8) prototyped methods proves to be 10x times slower than functions, this can be ignored for this case.

All this may be related only to a String object, because simple generic objects perform about the same when you pass them to functions or call their methods:

var A = { count: 1234 };

function getCount(obj) { return obj.count }

A.getCount = function() { return this.count }

console.time('func')
for (var i = 0; i < 1e9; i++) getCount(A)
console.timeEnd('func')

console.time('method')
for (var i = 0; i < 1e9; i++) A.getCount()
console.timeEnd('method')

Results:

func: 1689.942ms
method: 1674.639ms

I've been searching on Stackoverflow and binging, but other that the remendation "do not extend String or Array because you pollute the name space" (which is not a problem for my particular project), I cannot find anything related to performance of methods pared to functions. So should I simply forget about extending the String object due to performance drop of added methods or there is more about it?

• javascript
• performance

Share Improve this question Follow asked Jul 16, 2016 at 4:36 exebookexebook 34.1k4242 gold badges152152 silver badges241241 bronze badges 1

• Thanks for your information but, you're last implementation is far away from prototype you can replace object with function and then add prototype method. then get a new instance of it. BTW result is the same. – Morteza Tourani Commented Jul 16, 2016 at 5:37

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1 Answer 1

Sorted by: Reset to default 10

This is most likely because you are not using strict mode, and the this value inside your method is getting coerced to a String instance instead of being a primitive string. This coercion, and further method calls or property accesses on the String object, are slower than using primitive values.

You can (Edit: could, at least, in 2016) confirm this by repeating your measurement on var STR = new String('01101011…') which should have less overhead.

Then fix your implementation:

String.prototype.count = function (char) {
    "use strict";
//  ^^^^^^^^^^^^
    var n = 0;
    for (var i = 0; i < this.length; i++)
        if (this[i] == char)
            n++;
    return n;
};

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