let, const, types & coercion

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var oldStyle = 1;     // function-scoped, hoisted, avoid
let canChange = 2;    // block-scoped, can reassign
const fixed = 3;      // block-scoped, can't reassign

canChange = 20;       // OK
// fixed = 30;        // TypeError: Assignment to constant variable

const arr = [1, 2];
arr.push(3);          // OK — const only blocks reassignment, not mutation
console.log(arr);     // [1, 2, 3]

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// Falsy values — the whole list
const falsy = [false, 0, -0, '', null, undefined, NaN];

for (const v of falsy) {
    if (!v) console.log(JSON.stringify(v), 'is falsy');
}

// Everything else is truthy — including:
console.log(!![]);        // true — empty array is truthy
console.log(!!{});        // true — empty object is truthy
console.log(!!"false");   // true — the string "false" is truthy

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// + with a string concatenates
console.log(1 + "2");      // "12"
console.log("a" + 1 + 2);  // "a12"
console.log(1 + 2 + "a");  // "3a"

// Other operators coerce to number
console.log("5" - 1);      // 4
console.log("5" * "2");    // 10

// Boolean context coerces with !!
console.log(!!"hi");        // true
console.log(!!0);           // false