"const、let 和 var 对 v8 JavaScript 性能的影响？"

Question

Regardless of functional differences, does using the new keywords 'let' and 'const' have any generalized or specific impact on performance relative to 'var'?

After running the program:

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 results were the following:

ctl = {"min":101,"max":117,"mean":108.34,"spread":4.145407097016924}
con = {"min":107,"max":572,"mean":435.7,"spread":169.4998820058587}
var = {"min":103,"max":608,"mean":439.82,"spread":176.44417700791374}

However discussion as noted here seems to indicate a real potential for performance differences under certain scenarios: https://esdiscuss.org/topic/performance-concern-with-let-const

Answer 1

TL;DR

In theory , an unoptimized version of this loop:

for (let i = 0; i < 500; ++i) {
    doSomethingWith(i);
}

might be slower than an unoptimized version of the same loop with var :

for (var i = 0; i < 500; ++i) {
    doSomethingWith(i);
}

because a different i variable is created for each loop iteration with let , whereas there's only one i with var .

Arguing against that is the fact the var is hoisted so it's declared outside the loop whereas the let is only declared within the loop, which may offer an optimization advantage.

In practice , here in 2018, modern JavaScript engines do enough introspection of the loop to know when it can optimize that difference away. (Even before then, odds are your loop was doing enough work that the additional let -related overhead was washed out anyway. But now you don't even have to worry about it.)

Beware synthetic benchmarks as they are extremely easy to get wrong, and trigger JavaScript engine optimizers in ways that real code doesn't (both good and bad ways). However, if you want a synthetic benchmark, here's one:

 const now = typeof performance === "object" && performance.now ? performance.now.bind(performance) : Date.now.bind(Date); const btn = document.getElementById("btn"); btn.addEventListener("click", function() { btn.disabled = true; runTest(); }); const maxTests = 100; const loopLimit = 50000000; const expectedX = 1249999975000000; function runTest(index = 1, results = {usingVar: 0, usingLet: 0}) { console.log(`Running Test #${index} of ${maxTests}`); setTimeout(() => { const varTime = usingVar(); const letTime = usingLet(); results.usingVar += varTime; results.usingLet += letTime; console.log(`Test ${index}: var = ${varTime}ms, let = ${letTime}ms`); ++index; if (index <= maxTests) { setTimeout(() => runTest(index, results), 0); } else { console.log(`Average time with var: ${(results.usingVar / maxTests).toFixed(2)}ms`); console.log(`Average time with let: ${(results.usingLet / maxTests).toFixed(2)}ms`); btn.disabled = false; } }, 0); } function usingVar() { const start = now(); let x = 0; for (var i = 0; i < loopLimit; i++) { x += i; } if (x !== expectedX) { throw new Error("Error in test"); } return now() - start; } function usingLet() { const start = now(); let x = 0; for (let i = 0; i < loopLimit; i++) { x += i; } if (x !== expectedX) { throw new Error("Error in test"); } return now() - start; }

 <input id="btn" type="button" value="Start">

It says that there's no significant difference in that synthetic test on either V8/Chrome or SpiderMonkey/Firefox. (Repeated tests in both browsers have one winning, or the other winning, and in both cases within a margin of error.) But again, it's a synthetic benchmark, not your code. Worry about the performance of your code when and if your code has a performance problem.

As a style matter, I prefer let for the scoping benefit and the closure-in-loops benefit if I use the loop variable in a closure.

Details

The important difference between var and let in a for loop is that a different i is created for each iteration; it addresses the classic "closures in loop" problem:

 function usingVar() { for (var i = 0; i < 3; ++i) { setTimeout(function() { console.log("var's i: " + i); }, 0); } } function usingLet() { for (let i = 0; i < 3; ++i) { setTimeout(function() { console.log("let's i: " + i); }, 0); } } usingVar(); setTimeout(usingLet, 20);

Creating the new EnvironmentRecord for each loop body ( spec link ) is work, and work takes time, which is why in theory the let version is slower than the var version.

But the difference only matters if you create a function (closure) within the loop that uses i , as I did in that runnable snippet example above. Otherwise, the distinction can't be observed and can be optimized away.

Here in 2018, it looks like V8 (and SpiderMonkey in Firefox) is doing sufficient introspection that there's no performance cost in a loop that doesn't make use of let 's variable-per-iteration semantics. See this test .

---

In some cases, const may well provide an opportunity for optimization that var wouldn't, especially for global variables.

The problem with a global variable is that it's, well, global; any code anywhere could access it. So if you declare a variable with var that you never intend to change (and never do change in your code), the engine can't assume it's never going to change as the result of code loaded later or similar.

With const , though, you're explicitly telling the engine that the value cannot change¹. So it's free to do any optimization it wants, including emitting a literal instead of a variable reference to code using it, knowing that the values cannot be changed.

¹ Remember that with objects, the value is a reference to the object, not the object itself. So with const o = {} , you could change the state of the object ( o.answer = 42 ), but you can't make o point to a new object (because that would require changing the object reference it contains).

---

When using let or const in other var -like situations, they're not likely to have different performance. This function should have exactly the same performance whether you use var or let , for instance:

function foo() {
    var i = 0;
    while (Math.random() < 0.5) {
        ++i;
    }
    return i;
}

---

It's all, of course, unlikely to matter and something to worry about only if and when there's a real problem to solve.

Answer 2

"LET" IS BETTER IN LOOP DECLARATIONS

With a simple test (5 times) in navigator like that:

// WITH VAR
console.time("var-time")
for(var i = 0; i < 500000; i++){}
console.timeEnd("var-time")

The mean time to execute is more than 2.5ms

// WITH LET
console.time("let-time")
for(let i = 0; i < 500000; i++){}
console.timeEnd("let-time")

The mean time to execute is more than 1.5ms

I found that loop time with let is better.

Answer 3

TJ Crowder 's answer is so excellent.

Here is an addition of: "When would I get the most bang for my buck on editing existing var declarations to const ?"

I've found that the most performance boost had to do with "exported" functions.

So if file A, B, R, and Z are calling on a "utility" function in file U that is commonly used through your app, then switching that utility function over to "const" and the parent file reference to a const can eak out some improved performance. It seemed for me that it wasn't measurably faster, but the overall memory consumption was reduced by about 1-3% for my grossly monolithic Frankenstein-ed app. Which if you're spending bags of cash on the cloud or your baremetal server, could be a good reason to spend 30 minutes to comb through and update some of those var declarations to const.

I realize that if you read into how const, var, and let work under the covers you probably already concluded the above... but in case you "glanced" over it :D.

From what I remember of the benchmarking on node v8.12.0 when I was making the update, my app went from idle consumption of ~240MB RAM to ~233MB RAM.

Answer 4

TJ Crowder's answer is very good but :

1. 'let' is made to make code more readable, not more powerful
2. by theory let will be slower than var
3. by practice the compiler can not solve completely (static analysis) an uncompleted program so sometime it will miss the optimization
4. in any-case using 'let' will require more CPU for introspection, the bench must be started when google v8 starts to parse
5. if introspection fails 'let' will push hard on the V8 garbage collector, it will require more iteration to free/reuse. it will also consume more RAM. the bench must take these points into account
6. Google Closure will transform let in var...

The effect of the performance gape between var and let can be seen in real-life complete program and not on a single basic loop.

Anyway, to use let where you don't have to, makes your code less readable.

Answer 5

As @Amn's test concludes, using let in a single for() loop is better. I achieved the same results after doing the same test.

console.time("let")
for(let i = 0; i < 500000; i++){}
console.timeEnd("let") // let: 2.8828125 ms

console.clear()

console.time("var")
for(var i = 0; i < 500000; i++){}
console.timeEnd("var") // 3.93896484375 ms

// let is faster

However, I have also tried using multiple let s in different loops .

Multiple i variables

console.time("var");
for(var i = 0; i < 10; i++)
 for(var i = 0; i < 10; i++)
  for(var i = 0; i < 10; i++)
   for(var i = 0; i < 10; i++);
console.timeEnd("var"); // var: 0.02734375 ms

console.clear()

console.time("let");
for(let i = 0; i < 10; i++)
 for(let i = 0; i < 10; i++)
  for(let i = 0; i < 10; i++)
   for(let i = 0; i < 10; i++);
console.timeEnd("let"); // let: 0.221923828125 ms

// var is faster

After this test, using let was on average 10 times slower.

Different variable names

console.time("var");
for(var i = 0; i < 10; i++)
 for(var j = 0; j < 10; j++)
  for(var k = 0; k < 10; k++)
   for(var l = 0; l < 10; l++);
console.timeEnd("var"); // var: 0.639892578125 ms

console.clear()

console.time("let");
for(let i = 0; i < 10; i++)
 for(let j = 0; j < 10; j++)
  for(let k = 0; k < 10; k++)
   for(let l = 0; l < 10; l++);
console.timeEnd("let"); // let: 0.180908203125 ms

// let is faster

There are advantages and disadvantages to both. Personally, I prefer let .

---

for...of

I also tested all three in a for...of loop. I used this test:

var arr = Array(1000).fill(0)
for(let i = 00; i < 100; i++){
  console.time("const")
  for(const e of arr);
  console.timeEnd("const")
}

I tried for(const...of) , for(let...of) , and for(var...of) . I took the average of all the times.

• let : 0.023911133 ms
• var : 0.023691406 ms
• const : 0.020488281 ms

The results of let and var are pretty similar. Even though var performed 0.00022 ms faster, I think it's safe to say that in this situation, it doesn't matter which one you use. As for const , it is clear that it is slightly faster.

So, when using for...of loops, I recommend using const But keep in mind, using const...of does not allow variable manipulation (but you probably already knew that).

Answer 6

just did some more tests and it seems that there is a substantial difference in favor of var<\/strong> . My results show that between Const \/ Let \/ Var there is a ratio from 4 \/ 4 \/ 1 to 3 \/ 3 \/ 1. I give the code i used below. Just let me mention that i started from the code of AMN<\/a> and did lots of tweaking. I did the tests both in w3schools_tryit editor and in Google_scripts My Notes:

Answer 7

code with 'let' will be more optimized than 'var' as variables declared with var do not get cleared when the scope expires but variables declared with let does. so var uses more space as it makes different versions when used in a loop.