Fundamentals: variables, conditions and loops

Variables and data types

A variable associates a name with a value. JavaScript let allows reassignment; const prevents reassigning that reference. Numbers support calculations, strings represent text and booleans express true or false. The string "12" and the number 12 have different types.

Conditions: choosing a path

if evaluates an expression to select a block; else supplies an alternative. Strict equality === compares without converting operands to another type, so 12 === "12" is false. Choose names that explain what you are checking.

const stock = 3;
if (stock > 0) {
  console.log("Available");
} else {
  console.log("Sold out");
}

Loops: repeating with a limit

for...of visits the values in a list. With while, ensure the condition can eventually become false; otherwise the program can enter an infinite loop.

const points = [4, 7, 2];
let total = 0;
for (const value of points) {
  total += value;
}
console.log(total); // 13

Reading the example

total starts at zero and accumulates one value per iteration. Check the output manually: 4 + 7 + 2 is 13. Resetting total inside the loop would discard the previous additions.

Practice and verification

1. Replace the list with [5, 0, 5] and predict the output first.
2. Try an empty list: the total should remain zero.
3. Add a condition to include only positive values, then try a negative number.