Functions — Declarations, Expressions, Arrow

9. Practice Problems

1. Write a function square in three different ways (declaration, expression, arrow).
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   a1.js

   function sq1(n) { return n * n; }
   const sq2 = function (n) { return n * n; };
   const sq3 = n => n * n;
   console.log(sq1(5), sq2(5), sq3(5));

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2. Compute the average of any number of arguments.
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   a2.js

   const avg = (...nums) =>
       nums.reduce((a, b) => a + b, 0) / nums.length;
   console.log(avg(10, 20, 30));   // 20

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3. Write greet(name, msg = "হ্যালো").
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   a3.js

   const greet = (name, msg = "হ্যালো") => `${msg}, ${name}!`;
   console.log(greet("Arif"));
   console.log(greet("Nusrat", "Welcome"));

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4. Build a function that takes a function and a number, returning the function applied n times.
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   a4.js

   const repeat = (fn, n) => x => {
       let r = x;
       for (let i = 0; i < n; i++) r = fn(r);
       return r;
   };
   const add5twice = repeat(x => x + 5, 2);
   console.log(add5twice(3)); // 13

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5. Show that arrow function inherits this by using setTimeout inside an object method.
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   a5.js

   const obj = {
       name: "Arif",
       say() {
           setTimeout(() => console.log(this.name), 0);
       }
   };
   obj.say();   // "Arif"

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6. Build an add that supports add(2)(3) currying.
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   a6.js

   const add = a => b => a + b;
   console.log(add(2)(3));   // 5
   const add5 = add(5);
   console.log(add5(10));    // 15

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7. Show that calling a function declaration above its definition works, but calling an arrow above doesn't.
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   a7.js

   declared();
   function declared() { console.log("hoisted!"); }
   
   try { arrow(); }
   catch (e) { console.log("err:", e.message); }
   const arrow = () => console.log("never");

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8. Write an arrow that returns an object literal.
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   a8.js

   const mkUser = name => ({ name, ts: Date.now() });
   console.log(mkUser("Arif"));

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9. Write a higher-order once that ensures a function runs at most once.
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   a9.js

   const once = fn => {
       let done = false, val;
       return (...args) => {
           if (!done) { val = fn(...args); done = true; }
           return val;
       };
   };
   const init = once(() => { console.log("init!"); return 42; });
   init(); init(); init();   // "init!" only once

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10. Build a function that returns its own arity (number of declared params).
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    a10.js

    function f(a, b, c) {}
    console.log(f.length);    // 3
    
    const g = (a, b = 5, ...rest) => {};
    console.log(g.length);    // 1 (stops before defaults/rest)

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11. Use IIFE to compute pi using a series and return the value.
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    a11.js

    const pi = (() => {
        let s = 0;
        for (let i = 0; i < 100000; i++)
            s += (i % 2 ? -1 : 1) / (2 * i + 1);
        return s * 4;
    })();
    console.log(pi);

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12. Why can't you use new on an arrow function?
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    Answer: Arrow functions don't have their own this binding, no prototype property, and no [[Construct]] internal slot — calling them with new throws "X is not a constructor". Use a regular function or a class for object construction.

13. Use rest parameters and the spread operator together.
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    a13.js

    const max = (...n) => Math.max(...n);
    const nums = [3, 9, 1, 7];
    console.log(max(...nums));   // 9

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14. Build a function memo that caches results by argument.
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    a14.js

    const memo = fn => {
        const cache = new Map();
        return x => cache.has(x) ? cache.get(x) :
            (cache.set(x, fn(x)), cache.get(x));
    };
    const slow = n => { console.log("compute", n); return n * n; };
    const fast = memo(slow);
    console.log(fast(3));  // compute 3, 9
    console.log(fast(3));  // 9 (cached)

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15. Show that arguments doesn't exist inside an arrow function.
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    a15.js

    function classic() { console.log(arguments.length); }
    classic(1, 2, 3);   // 3
    
    const arr = (...args) => console.log(args.length);
    arr(1, 2, 3);       // 3 (use rest)

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16. Write a tag-aware logger: logger("DB")(message).
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    a16.js

    const logger = tag => msg => console.log(`[${tag}]`, msg);
    const dbLog = logger("DB");
    dbLog("connected");
    dbLog("query OK");

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