JavaScript Loops

A loop repeats a block of code many times, so you do not write the same line again and again.

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for loop

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

The for has three parts: start (let i = 1), condition (i <= 5) and step (i++).

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while and do while

let n = 3;
while (n > 0) {
  console.log(n);
  n--;
}

let x = 10;
do {
  console.log("Runs once, x =", x);
} while (x < 5);

while checks the condition first. do while runs the block at least once, and then checks.

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for…of (arrays and strings)

const fruits = ["apple", "banana", "mango"];

for (const fruit of fruits) {
  console.log(fruit);
}

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for…in (object keys)

const user = { name: "Robin", age: 25 };

for (const key in user) {
  console.log(key + ": " + user[key]);
}

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break and continue

for (let i = 1; i <= 6; i++) {
  if (i === 3) continue;   // skip 3
  if (i === 5) break;      // stop at 5
  console.log(i);
}

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Summary

• for is for a known number of rounds, while for an unknown number.
• for...of loops over values, for...in over object keys.
• break stops the loop, continue skips one round.
• Next: JavaScript Functions.

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