Write-up for Leetcode problem 2635. Apply Transform Over Each Element in Array

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/README.md

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+# 2635. Apply Transform Over Each Element in Array
+
+[View Problem on LeetCode](https://leetcode.com/problems/apply-transform-over-each-element-in-array/)
+
+We're about to get into a bunch of problems that essentially ask us to implement some built-in JavaScript function. This problem is asking us to implement mapping without using [`.map`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map), so of course the first thing we should do is get accepted using `.map`!
+
+For a more serious solution, we can keep it simple with some kind of loop that builds the result.
+
+One more thing worth practicing is implementing `.map` using [`.reduce`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce). Since pretty much anything can be done through `.reduce`, it was at one point popular in interviews to ask candidates to build some other operation without explicit loops, instead using only `.reduce`.
+
+It's also possible to consider other built-in functions. For example, LeetCode didn't ask us not to use [`.flatMap`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap)!
+
+Once you've worked on the problem, check out [the full write-up and solution](solution.md)!

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/iterative-loops/using-classic-for.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] {
+  const res: TOut[] = [];
+
+  for (let i = 0; i < arr.length; ++i) {
+    res.push(fn(arr[i], i));
+  }
+
+  return res;
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/iterative-loops/using-for-each.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] {
+  const res: TOut[] = [];
+
+  arr.forEach((element, index) => {
+    res.push(fn(element, index));
+  });
+
+  return res;
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/iterative-loops/using-for-of-entries.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] {
+  const res: TOut[] = [];
+
+  for (const [index, element] of arr.entries()) {
+    res.push(fn(element, index));
+  }
+
+  return res;
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/iterative-loops/using-generators.ts

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+function* lazyMap<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): Generator<TOut, void, void> {
+  for (const [index, element] of arr.entries()) {
+    yield fn(element, index);
+  }
+}
+
+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] => Array.from(lazyMap(arr, fn));

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/recursive/inner-function-with-index-argument.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] {
+  const res: TOut[] = [];
+
+  const doMap = (index: number) => {
+    if (index === arr.length) {
+      return;
+    }
+
+    res.push(fn(arr[index], index));
+    doMap(index + 1);
+  };
+
+  doMap(0);
+
+  return res;
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/recursive/inner-function-without-index-argument.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] {
+  const res: TOut[] = [];
+
+  const doMap = () => {
+    if (res.length === arr.length) {
+      return;
+    }
+
+    res.push(fn(arr[res.length], res.length));
+    doMap();
+  };
+
+  doMap();
+
+  return res;
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/recursive/single-function-with-default-argument.ts

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+function map<TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+  res: TOut[] = [],
+): TOut[] {
+  if (res.length === arr.length) {
+    return res;
+  }
+
+  res.push(fn(arr[res.length], res.length));
+  return map(arr, fn, res);
+}

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-built-in-map/binding.js

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+/**
+ * @param {number[]} arr
+ * @param {Function} fn
+ * @return {number[]}
+ */
+const map = Function.prototype.call.bind(Array.prototype.map);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-built-in-map/delegating.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] => arr.map(fn);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-other-mapping-built-ins/aliasing-array-from.js

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+/**
+ * @param {number[]} arr
+ * @param {Function} fn
+ * @return {number[]}
+ */
+const map = Array.from;

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-other-mapping-built-ins/delegating-to-array-from.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] => Array.from(arr, fn);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-other-mapping-built-ins/delegating-to-flat-map-safe.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] => arr.flatMap((element, index) => [fn(element, index)]);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-other-mapping-built-ins/delegating-to-flat-map-sloppy.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] => arr.flatMap(fn);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-reduce/elegant-but-quadratic.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] =>
+  arr.reduce((res: TOut[], element, index) => [...res, fn(element, index)], []);

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-reduce/linear-contrarian.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] =>
+  arr
+    .reduceRight((res: TOut[], element, index) => {
+      res.push(fn(element, index));
+      return res;
+    }, [])
+    .reverse();

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-reduce/linear-with-comma-operator.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] =>
+  arr.reduce(
+    (res: TOut[], element, index) => (res.push(fn(element, index)), res),
+    [],
+  );

workspaces/javascript-leetcode-month/problems/2635-apply-transform-over-each-element-in-array/solutions/using-reduce/linear.ts

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+const map = <TIn, TOut>(
+  arr: readonly TIn[],
+  fn: (element: TIn, index: number) => TOut,
+): TOut[] =>
+  arr.reduce((res: TOut[], element, index) => {
+    res.push(fn(element, index));
+    return res;
+  }, []);