A detailed introduction to for/of, for/in in JavaScript

In JavaScript , there are several common ways to write for loops:

The first and most common way of writing:

nums = [1,2,3,4]

for (let i=0; i<nums.length; i++){
    console.log(nums[i])
}

Second type:

The second for/of syntax is available since ES6 . It can directly iterate each element in the array without getting the element by subscript index position. In fact, as long as it is an iterable object, for/of can be used.

for (let item of nums){
    console.log(item)
}

The third type:

The third writing method for/in . Unlike for/of must be iterable objects, for/in can iterate over any object. The name of the property of the loop iteration object. If it is an array, the iterated value is the subscript index of the array, which is the same as the original for .

let p = {name:"zhang", age:10}

for(let key in p){
    console.log(p[key])
}

Output:

zhang
10

for (let index in nums){
    console.log(nums[index])
}

for/in cannot enumerate all the properties of the iterated object, such as symbol properties cannot be enumerated

When defining variables for/of and for/in syntax, you can also use the const keyword. const declares a constant value during a loop iteration.

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