mrmartineau/array_iteration_thoughts.md

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Array iteration methods summarized

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array_iteration_thoughts.md

While attempting to explain JavaScript's reduce method on arrays, conceptually, I came up with the following - hopefully it's helpful; happy to tweak it if anyone has suggestions.

forEach:

callback answers: here’s an item. do something nutty with it, i don't care what.
callback gets these arguments: item, index, list
final return value: nothing - in other words, undefined

map:

callback answers: here’s an item. what should i replace it with?
callback gets these arguments: item, index, list
final return value: list of new items

filter:

callback is a predicate - it should return a truthy or falsy value
callback answers: should i keep this item?
callback gets these arguments: item, index, list
final return value: list of kept items

reduce:

callback answers: here’s the result from the previous iteration. what should i pass to the next iteration?
callback gets these arguments: result, item, index, list
final return value: result of last iteration

reduceRight: (same as reduce, but in reversed order: last-to-first)

some:

callback is a predicate - it should return a truthy or falsy value
callback answers: does this item meet your criteria?
callback gets these arguments: item, index, list
final return value: true after the first item that meets your criteria, else false

every:

callback is a predicate - it should return a truthy or falsy value
callback answers: does this item meet your criteria?
callback gets these arguments: item, index, list
final return value: false after the first item that failed to meet your criteria, else true

find:

callback is a predicate - it should return a truthy or falsy value
callback answers: is this item what you’re looking for?
callback gets these arguments: item, index, list
final return value: the item you’re looking for, or undefined

findIndex:

callback is a predicate - it should return a truthy or falsy value
callback answers: is this item what you’re looking for?
callback gets these arguments: item, index, list
final return value: the index of the item you’re looking for, or -1

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