Talk about the bugs in using for in to traverse the array in js

*****For in array traversal pit*****

Using for in traversal on an array, we will encounter a very serious bug, that is, for in will traverse the properties added to the prototype chain of the array, that is to say, the for-in loop body acting on the arrayIn addition to traversing array elements, custom properties are also traversed.

1. When we use for in to traverse the array

Array.prototype.istrue = function(value) {return true;}

var a = [1,2];

for(var i in a) {

console.log(a[i]);

}

Output:

1

2

function(value) {return true;}

Therefore, it is not recommended to use for in in array traversal, but use forEach or for of instead, because forEach can only traverse index arrays, while for of can traverse everything with numerical subscripts, that is, index arrays, array-like objects,String, so we generally recommend to use for of instead of forEach and for in to traverse the array.Note: The i variable above is a string, not a number

2. Generally, we usually use for in to traverse objects

var a = {"x":1,"y":2};

for(var i in a) {

console.log(a[i]);}

Output:

1

2

3. If you use for in to traverse the array, use hasOwnProperty to avoid traversing properties or methods on the prototype chain

Array.prototype.istrue = function(value) {return true;}

var a = [1,2];

for(var i in a) {

if(a.hasOwnProperty(i){

console.log(a[i]);

})

}

Output:

1

2