javascript tutorial - [5 Solutions] “for…in” - javascript - java script - javascript array
Problem
Why is using “for…in” with array iteration a bad idea?
Solution 1:
The reason is that one construct:
var a = []; // Create a new empty array.
a[5] = 5; // Perfectly legal JavaScript that resizes the array.
for (var we = 0; we < a.length; i++) {
// Iterate over numeric indexes from 0 to 5, as everyone expects.
console.log(a[i]);
}
/* Will display:
undefined
undefined
undefined
undefined
5
*/
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Solution 2:
can sometimes be totally different from the other:
var a = [];
a[5] = 5;
for (var x in a) {
// Shows only the explicitly set index of "5", and ignores 0-4
console.log(x);
}
/* Will display:
5
*/
click below button to copy the code. By JavaScript tutorial team
Solution 3:
Also consider that JavaScript libraries might do things like this, which will affect any array we create:
// Somewhere deep in our JavaScript library...
Array.prototype.foo = 1;
// Now we have no idea what the below code will do.
var a = [1, 2, 3, 4, 5];
for (var x in a){
// Now foo is a part of EVERY array and
// will show up here as a value of 'x'.
console.log(x);
}
/* Will display:
0
1
2
3
4
foo
*/
click below button to copy the code. By JavaScript tutorial team
Solution 4:
Because for...in enumerates through the object that holds the array, not the array itself. If WE add a function to the arrays prototype chain, that will also be included. I.e.
Array.prototype.myOwnFunction = function() { alert(this); }
a = new Array();
a[0] = 'foo';
a[1] = 'bar';
for(x in a){
document.write(x + ' = ' + a[x]);
}
This will write:
0 = foo
1 = bar
myOwnFunction = function() { alert(this); }
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And since we can never be sure that nothing will be added to the prototype chain just use a for loop to enumerate the array:
for(i=0,x=a.length;i<x;i++){
document.write(we + ' = ' + a[i]);
}
This will write:
0 = foo
1 = bar
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Solution 5:
In addition to the other problems, the "for..in" syntax is probably slower, because the index is a string, not an integer.
var a = ["a"]
for (var we in a)
alert(typeof i) // 'string'
for (var we = 0; we < a.length; i++)
alert(typeof i) // 'number'
click below button to copy the code. By JavaScript tutorial team
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