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JavaScript: Scripting language
Web APIs: Programming interfaces
Structuring content with HTML module
Dynamic scripting with JavaScript module
It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments. These habits may lead to
It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments. These habits may lead to
Mapped array contains undefined
When undefined or nothing is returned, the resulting array contains undefined. If you want to delete the element instead, chain a filter() method, or use the flatMap() method and return an empty array to signify deletion
Mapped array contains undefined
When undefined or nothing is returned, the resulting array contains undefined. If you want to delete the element instead, chain a filter() method, or use the flatMap() method and return an empty array to signify deletion
Mapped array contains undefined
When undefined or nothing is returned, the resulting array contains undefined. If you want to delete the element instead, chain a filter() method, or use the flatMap() method and return an empty array to signify deletion
Calling map() on non-array objects
The map() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.
This page was last modified on Jul 12, 2026 by MDN contributors.