keyboard_row.js

const rows = [
  "qwertyuiop",
  "asdfghjkl",
  "zxcvbnm",
].reduce(
  (acc, curr, i) => {
    curr.split('').forEach(c => acc[c] = i);
    return acc;
  },
  {},
);

/**
 * Returns a boolean showing whether the word `s` can be typed on a single row
 * given a standard ANSI layout keyboard.
 *
 * @param s string
 * @return bool
 */
export function singleRow(s) {
  let curr = rows[s[0]];
  
  for (let c of s.split('')) {
    if (curr != rows[c]) {
      return false;
    }
  }

  return true;
}

if (import.meta.main) {
  console.log(Deno.args[0]);
  console.log(singleRow(Deno.args[0] ?? ""));
}

keyboard_row_bench.js

import { singleRow } from "./keyboard_row.js";

// Assumes MacOS for the location of the dictionary file
const words = Deno.readTextFileSync("/usr/share/dict/web2").split('\n');

Deno.bench("singleRow", () => {
  for (let w of words) {
    singleRow(w);
  }
});

// Results
// cpu: Apple M1 Pro
// runtime: deno 1.34.1 (aarch64-apple-darwin)
//
// file:///REDACTED/keyboard-row_test.js
// benchmark      time (avg)             (min … max)       p75       p99      p995
// ------------------------------------------------- -----------------------------
// singleRow  11.23 ms/iter(11.11 ms … 11.64 ms) 11.26 ms 11.64 ms 11.64 ms

keyboard_row_test.js

import { singleRow } from "./keyboard_row.js";
import { assert, assertFalse } from "https://deno.land/std@0.190.0/testing/asserts.ts";

Deno.test("single row words", () => {
  assert(singleRow("typewriter"));
});

Deno.test("multi row words", () => {
  assertFalse(singleRow("fantastic"));
});