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JavaScript JavaScript Objects Loop Through Objects Use `for in` to Loop Through an Object's Properties

Difference between " for in " and " for loop "

what is the Difference between " for in " and " for loop "

Somehow they look the same but upon my research, some is saying to not use for in for arrays?

Trent Stenoien
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Trent Stenoien
Full Stack JavaScript Techdegree Graduate 20,034 Points

They serve a similar purpose, but a 'for' loop are for arrays while 'for...in' loops are for objects. In the 'for' loop below, you see we set i = 0, then increment by one while i < array.length. We can then call 'array[i]' as we go along. Technically, arrays are objects in JS though so 'for...in' loops work on arrays too. I don't know the performance differences, but I like how many more options there are in 'for' loops so that's what I tend to use.

for (let i = 0; i < array.length; i++) { ... } for (const property in object) { ... }

1 Answer

Steven Parker
Steven Parker
220,683 Points

Perhaps the main difference is that a standard "for" loop will return the index as a number, but a "for in" will return it as a string. This is the primary reason that the standard "for" is recommended when iterating arrays.

Thank you Steven. From your experience, how often do you do you see people use " for in " in real projects? I understand " for " very well and plan to stick by it. Wondering if " for in " is something I should be too concerned about and tackle that or not.

Steven Parker
Steven Parker
220,683 Points

The "for in" loop is used all the time with objects in real projects, but typically not with arrays. Now another loop type that is used with arrays is "for of".