JavaScript essentials for Sheets users

Concept

1. Variables

const gstRate = 0.03;     // const: value won't change
let total = 0;            // let: value can change
total = total + 500;

Use const by default, let when the value must change. Avoid the old var.

2. Data types

Type	Example	Sheet equivalent
Number	`1250.5`	A numeric cell
String	`'Ring'`	A text cell
Boolean	`true` / `false`	A checkbox
Date	`new Date()`	A date cell
Empty	`''`	A blank cell (Sheets returns `''`, not `null`)

Template literals make text easy:

const name = 'Rahul';
const amount = 4500;
const msg = `Dear ${name}, your bill is ₹${amount}.`;

3. Functions

A function is a reusable block of steps — like a formula you write yourself.

function addGst(amount, rate) {
  return amount + amount * rate;
}

const finalAmount = addGst(10000, 0.03); // 10300

Arrow-function shorthand (you'll see it in examples):

const addGstShort = (amount, rate) => amount * (1 + rate);

4. Arrays — and how a sheet looks in code

An array is an ordered list:

const items = ['Ring', 'Chain', 'Bangle'];
console.log(items[0]);      // 'Ring'  (counting starts at 0!)
console.log(items.length);  // 3

When you read a range from a sheet, you get an array of rows, and each row is an array of cells:

// Range A1:C3 comes back as:
const data = [
  ['Item',   'Qty', 'Price'],   // row 1 → data[0]
  ['Ring',    2,     15000],    // row 2 → data[1]
  ['Chain',   1,     42000]     // row 3 → data[2]
];

console.log(data[1][0]); // 'Ring'  → row 2, column A
console.log(data[2][2]); // 42000   → row 3, column C

Watch out: sheet rows and columns start at 1, arrays start at 0. data[1][0] is cell A2.

5. Objects

An object groups named values — like one row with headings:

const order = { item: 'Ring', qty: 2, price: 15000 };
console.log(order.item);           // 'Ring'
console.log(order.qty * order.price); // 30000

6. Conditions

if (qty <= reorderLevel) {
  console.log('Reorder needed');
} else if (qty <= reorderLevel * 2) {
  console.log('Running low');
} else {
  console.log('Stock OK');
}

Comparison: === equal, !== not equal, >, <, >=, <=. Combine with && (and), || (or). Always use ===, not ==.

7. Loops

for (let i = 1; i < data.length; i++) {   // start at 1 to skip the header row
  const row = data[i];
  console.log(row[0] + ' → ' + row[1] * row[2]);
}

// Shorter, when you don't need the index:
for (const row of data.slice(1)) {
  console.log(row[0]);
}

8. Array helpers you'll use constantly

const rows = data.slice(1);                         // drop header

const expensive = rows.filter(r => r[2] > 20000);   // like FILTER()
const names = rows.map(r => r[0]);                  // pick one column
const totalValue = rows.reduce((sum, r) => sum + r[1] * r[2], 0); // like SUMPRODUCT()

Key points

Sheet data = array of rows; data[row][col], both starting at 0.
const by default, let when it changes, === for comparing.
filter, map, reduce replace most loops.

What you'll learn