For Loop

for-in VS Object.keys

for-in can access whole properties' enumerable name, including prototype.

Object.keys can only access object itself properties' enumerable name, not including prototype.

const obj = { a:1 };
const obj2 = Object.create(obj);
obj2.b = 2;

for (let i in obj2){
    console.log(i)  //b, a
}

console.log(Object.keys(obj2)); //["b"]

for-in VS for-of

for a key-value object, for-in will get its key, while key-of can get its value. For example, traverse an array:

const arr = [1,2,3];
for (let i in arr){
    console.log(i); //0,1,2
}
for (let item of arr){
    console.log(item);  //1,2,3
}

In fact, if it don't need to change the element of the array, forEach is better a choice. If it needs to change the element, map is better. forEach and map's second parameter thisArg can change this point in the callback.

let in for loop

In any for loop, use let to define the variable is always better than var. There is a typical example:

for (var i=0;i<5;i++){
    setTimeout(()=>{
        console.log(i); //5,5,5,5,5
    });
}
for (let i=0;i<5;i++){
    setTimeout(()=>{
        console.log(i); //0,1,2,3,4
    });
}

the performance impact of Nested sequence

var t1 = new Date().getTime()
for (let i = 0; i < 100; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 10000; k++) {
    }
  }
}
var t2 = new Date().getTime()
console.log('first time', t2 - t1)

for (let i = 0; i < 10000; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 100; k++) {

    }
  }
}
var t3 = new Date().getTime()
console.log('two time', t3 - t2)

Their loop times is the same, but their execution time is different.

The reason is that they have different variable initialization times:

• first loop:

  j: 100 k: 1000

• second loop:

  j: 10000 k: 10000 * 1000

Normally, larger surrounding loop, poorer performance.