no-loop-statements.md

Disallow imperative loops (functional/no-loop-statements)

💼 This rule is enabled in the following configs: ☑️ lite, noStatements, ✅ recommended, 🔒 strict.

This rule disallows for loop statements, including for, for...of, for...in, while, and do...while.

Rule Details

In functional programming we want everything to be an expression that returns a value. Loops in JavaScript are statements so they are not a good fit for a functional programming style. Instead consider using map, reduce or similar. For more background see this blog post and discussion in tslint-immutable #54.

❌ Incorrect

/* eslint functional/no-loop-statements: "error" */

const numbers = [1, 2, 3];
const double = [];
for (let i = 0; i < numbers.length; i++) {
  double[i] = numbers[i] * 2;
}

/* eslint functional/no-loop-statements: "error" */

const numbers = [1, 2, 3];
let sum = 0;
for (const number of numbers) {
  sum += number;
}

✅ Correct

/* eslint functional/no-loop-statements: "error" */
const numbers = [1, 2, 3];
const double = numbers.map((n) => n * 2);

/* eslint functional/no-loop-statements: "error" */

const numbers = [1, 2, 3];
const sum = numbers.reduce((carry, number) => carry + number, 0);