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V8 Javascript Speed comparison for "const" and "var" usage.

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js_const_var_speed.js
JavaScript
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function timeit(f, N, S) {
var start, timeTaken;
var stats = {min: 1e50, max: 0, N: 0, sum: 0, sqsum: 0};
var i;
for (i = 0; i < S; ++i) {
start = Date.now();
f(N);
timeTaken = Date.now() - start;
 
stats.min = Math.min(timeTaken, stats.min);
stats.max = Math.max(timeTaken, stats.max);
stats.sum += timeTaken;
stats.sqsum += timeTaken * timeTaken;
stats.N++
}
 
var mean = stats.sum / stats.N;
var sqmean = stats.sqsum / stats.N;
 
return {min: stats.min, max: stats.max, mean: mean, spread: Math.sqrt(sqmean - mean * mean)};
}
 
var variable1 = 10;
var variable2 = 10;
var variable3 = 10;
var variable4 = 10;
var variable5 = 10;
var variable6 = 10;
var variable7 = 10;
var variable8 = 10;
var variable9 = 10;
var variable10 = 10;
 
function varAccess(N) {
var i, sum;
for (i = 0; i < N; ++i) {
sum += variable1;
sum += variable2;
sum += variable3;
sum += variable4;
sum += variable5;
sum += variable6;
sum += variable7;
sum += variable8;
sum += variable9;
sum += variable10;
}
return sum;
}
 
const constant1 = 10;
const constant2 = 10;
const constant3 = 10;
const constant4 = 10;
const constant5 = 10;
const constant6 = 10;
const constant7 = 10;
const constant8 = 10;
const constant9 = 10;
const constant10 = 10;
 
function constAccess(N) {
var i, sum;
for (i = 0; i < N; ++i) {
sum += constant1;
sum += constant2;
sum += constant3;
sum += constant4;
sum += constant5;
sum += constant6;
sum += constant7;
sum += constant8;
sum += constant9;
sum += constant10;
}
return sum;
}
 
 
function control(N) {
var i, sum;
for (i = 0; i < N; ++i) {
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
sum += 10;
}
return sum;
}
 
console.log("ctl = " + JSON.stringify(timeit(control, 10000000, 50)));
console.log("con = " + JSON.stringify(timeit(constAccess, 10000000, 50)));
console.log("var = " + JSON.stringify(timeit(varAccess, 10000000, 50)));

My measurements using node.js v0.6.3 on MacOSX -

ctl = {"min":182,"max":187,"mean":183.8,"spread":0.98}
con = {"min":770,"max":789,"mean":774.92,"spread":3.16}
var = {"min":196,"max":253,"mean":224.42,"spread":5.96}

(The spread rounded to two decimal places)

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