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

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

运行程序后:

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)));

.. 我的结果如下:

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}

然而，这里提到的讨论似乎表明在某些情况下可能存在性能差异:https://esdiscuss.org/topic/performance-concern-with-let-const

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

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

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">

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);

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