Beginner to hired — in runnable code.
Pick a language, run real code in the browser, and get graded as you go — backed by mentors across every speciality. You write working code from the very first lesson.
// sum only the even numbers
const evenSum = (xs) =>
xs.filter((n) => n % 2 === 0)
.reduce((a, b) => a + b, 0);- evenSum([1,2,3,4]) → 6
- evenSum([]) → 0
- ignores the odd numbers
# sum the squares of the even numbers
def even_squares(nums):
return sum(n*n for n in nums if n % 2 == 0)- even_squares([1,2,3,4]) → 20
- even_squares([]) → 0
- skips the odd numbers
// turn a title into a URL slug
function slugify(string $t): string {
return preg_replace('/[^a-z0-9]+/', '-', strtolower(trim($t)));
}- slugify("Hello World") → "hello-world"
- trims the surrounding spaces
- collapses symbols to dashes
// Max returns the larger of two ints
func Max(a, b int) int {
if a > b { return a }
return b
}- Max(2, 9) → 9
- Max(-1, -4) → -1
- Max(5, 5) → 5
# count the palindromes in a list
def palindromes(words)
words.count { |w| w == w.reverse }
end- palindromes(["level","cat"]) → 1
- palindromes([]) → 0
- is case-sensitive
// an HTTP handler that echoes JSON
export default (req, res) => {
res.json({ ok: true, path: req.url });
};- responds with 200
- body has ok: true
- echoes the request path
JavaScript The language of the web — from the DOM to full front-end apps. 6 courses View courses
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JavaScript, from scratch
Variables to functions to the DOM — every concept run and checked in the browser.
Backend APIs with PHP
Routing, data, and a real JSON API — built the way production services actually are.
Python for data work
Clean, transform, and reason about data with a Python executor grading every step.
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No setup, no "works on my machine". Write code, run it, and see exactly what passed and what didn't — the same loop, every lesson.
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Always know what's next
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# reverse a string without slicing
def reverse(s):
out = ""
for ch in s:
out = ch + out
return out- reverse("code") → "edoc"
- reverse("") → ""
- reverse("a") → "a"
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