{"id":2289,"date":"2026-07-20T12:43:14","date_gmt":"2026-07-20T10:43:14","guid":{"rendered":"https:\/\/kindsonthegenius.com\/blog\/simple-example-of-test-driven-developmenttdd-in-javascript-step-by-step-tutorial\/"},"modified":"2026-08-26T21:23:15","modified_gmt":"2026-08-26T19:23:15","slug":"simple-example-of-test-driven-developmenttdd-in-javascript-step-by-step-tutorial","status":"publish","type":"post","link":"https:\/\/kindsonthegenius.com\/blog\/simple-example-of-test-driven-developmenttdd-in-javascript-step-by-step-tutorial\/","title":{"rendered":"Simple Example of Test-Driven Development(TDD) in JavaScript \u2013 Step by Step Tutorial"},"content":{"rendered":"

<\/p>\n

Updated August 2026 \u2014 full tutorial restored for this URL.<\/em><\/p>\n

Test-Driven Development (TDD)<\/strong> means write a failing test first, make it pass with the simplest code, then refactor. Here is a tiny JavaScript kata with Jest<\/strong>.<\/p>\n

C# twin: TDD in Visual Studio C#<\/a>.<\/p>\n

    \n
  1. Setup Jest<\/a><\/li>\n
  2. Red \u2013 failing test<\/a><\/li>\n
  3. Green \u2013 minimal code<\/a><\/li>\n
  4. Refactor<\/a><\/li>\n
  5. Add the next requirement<\/a><\/li>\n<\/ol>\n

    1. Setup Jest<\/strong><\/p>\n

    npm init -y\nnpm i -D jest\n# package.json \u2192 \"test\": \"jest\"\n<\/code><\/pre>\n
    Kata: sum(numbers: string): number<\/code> \u2014 empty string \u2192 0; comma-separated integers sum.<\/p>\n
    2. Red \u2013 failing test<\/strong><\/p>\n
    \/\/ sum.test.js\nconst { sum } = require(\".\/sum\");\n\ntest(\"empty string is 0\", () => {\n  expect(sum(\"\")).toBe(0);\n});\n<\/code><\/pre>\n
    npm test   # fails \u2014 module missing\n<\/code><\/pre>\n
    3. Green \u2013 minimal code<\/strong><\/p>\n
    \/\/ sum.js\nfunction sum(numbers) {\n  if (numbers === \"\") return 0;\n  return 0; \/\/ still enough for this one test\n}\nmodule.exports = { sum };\n<\/code><\/pre>\n
    4. Refactor<\/strong><\/p>\n
    Nothing much yet \u2014 keep tests green. Add the next test before inventing features.<\/p>\n
    5. Add the next requirement<\/strong><\/p>\n
    test(\"sums comma-separated numbers\", () => {\n  expect(sum(\"1,2,3\")).toBe(6);\n});\n<\/code><\/pre>\n
    function sum(numbers) {\n  if (numbers === \"\") return 0;\n  return numbers.split(\",\").map(Number).reduce((a, b) => a + b, 0);\n}\n<\/code><\/pre>\n
    Rhythm: red \u2192 green \u2192 refactor<\/strong>. Never pile features without a failing test that demands them.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Updated August 2026 \u2014 full tutorial restored for this URL. Test-Driven Development (TDD) means write a failing test first, make it pass with the simplest … <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"pagelayer_contact_templates":[],"_pagelayer_content":"","footnotes":""},"categories":[35],"tags":[],"class_list":["post-2289","post","type-post","status-publish","format-standard","hentry","category-algorithms"],"acf":[],"_links":{"self":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/2289","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/comments?post=2289"}],"version-history":[{"count":2,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/2289\/revisions"}],"predecessor-version":[{"id":2464,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/2289\/revisions\/2464"}],"wp:attachment":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/media?parent=2289"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/categories?post=2289"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/tags?post=2289"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}