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What's the Key Difference Between JavaScript's `for...in` and `for...of` Loops?
What's the Key Difference Between JavaScript's `for...in` and `for...of` Loops?
Dec 14, 2024 am 09:53 AM
Understanding the Distinction Between for...in and for...of Statements
In the realm of JavaScript looping, for...in and for...of statements often generate confusion. While for...in has been a cornerstone for iterating over properties, for...of emerged in ES6 to specifically target values. This article delves into the key differences between these two constructs.
for...in: Traversing Property Names
For...in loops iterate over the enumerable property names of an object. As demonstrated in the example, it successfully logs "0", "1", "2", and "foo", which represent the index keys of the array and the "foo" key of the array object.
for...of: Extracting Property Values
In contrast, for...of employs an object-specific iterator to traverse the values generated by that iterator. In the example, the array iterator yields only the values within the array, namely "3", "5", and "7". This behavior stems from the fact that iterators are designed to focus solely on values.
Non-Iteration of Non-Index Properties
Notably, for...of does not iterate over non-index properties. In the example, the "foo" property, which contains the value "hello", is not logged. This is because the array iterator only yields values associated with the array's indices.
Example: Value-Focused Iteration
To illustrate, the following loop should log the array values as well as the "foo" property value, but it logs only the array values:
for (var i of arr) {
console.log(i); // logs "3", "5", "7"
}
This behavior reinforces the understanding that for...of is purpose-built to iterate over values, excluding non-index properties.
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