Basic · B2

JavaScript basics

Apps Script is JavaScript. Before you control Google apps, you need the core of the language: storing values, working with them, making decisions and packing code into functions. Run every example in the editor and compare the log.

Variables are named boxes

A variable gives a value a name, so you can use it later. Modern JavaScript has two keywords:

const for a value that will not be reassigned. Use it by default.
let for a value you will change, such as a running total.

Code.gs

function variables() {
  const school = 'Suankularb Thonburi';  // const: will not change
  let score = 70;                         // let: can change
  score = score + 5;

  console.log(school);
  console.log(score);
  console.log(typeof school, typeof score);
}

Execution log

Suankularb Thonburi
75
string number

Old tutorials (and some code in the wild) use var. It still works, but it has confusing scope rules. Read it, but write const and let.

Naming rules

Use camelCase: studentName, lastRow, totalScore.
Names are case-sensitive: score and Score are different.
Comments start with // for one line, or go between /* and */.

Next: data types →

The kinds of value

Type	Example	In a sheet this is…
string	`'Ploy'`, `"M.5"`, `hi`	Any text cell
number	`82.5`, `-3`, `0`	A number cell (also percentages and money)
boolean	`true`, `false`	A ticked or unticked checkbox
Date	`new Date()`	A date or time cell
null / undefined	`null`	Never: an empty cell comes back as `''`, an empty string

Code.gs

function dataTypes() {
  const name = 'Ploy';          // string
  const mark = 82.5;            // number
  const passed = true;          // boolean
  const nothing = null;         // null: "no value on purpose"
  let notSet;                   // undefined: "never given a value"
  const today = new Date();     // a Date object

  console.log(typeof name, typeof mark, typeof passed);
  console.log(nothing, notSet);
  console.log(today.getFullYear());
}

Execution log

string number boolean
null undefined
2026

typeof tells you the type of a value. It is the first thing to check when a sheet gives you something you didn't expect.

Next: operators →

Doing things with values

Code.gs

function operators() {
  console.log(7 + 3, 7 - 3, 7 * 3, 7 / 2);  // 10 4 21 3.5
  console.log(7 % 3);                      // remainder: 1
  console.log(2 ** 5);                     // power: 32
  console.log('7' + 3);                    // joins text: 73
  console.log(Number('7') + 3);            // converts first: 10
  console.log(5 === 5, 5 === '5');         // true false
  console.log(5 > 3 && 2 > 9);             // and: false
  console.log(5 > 3 || 2 > 9);             // or: true
}

Execution log

10 4 21 3.5
1
32
73
10
true false
false
true

Operator	Meaning
`+ - * /`	Add, subtract, multiply, divide
`%`	Remainder after dividing. `n % 2 === 0` means n is even.
`**`	Power
`=== !==`	Equal / not equal, without converting types. Always use these.
`< > <= >=`	Comparisons
`&& \|\| !`	And, or, not
`+= -= ++`	Shortcuts: `total += 5` is `total = total + 5`

Template literals

Joining text with + gets messy. Backticks let you put values straight into text with $${…}:

Code.gs

function templates() {
  const name = 'Ploy';
  const mark = 82;
  console.log('Hello ' + name + ', you scored ' + mark + '%');
  console.log(`Hello ${name}, you scored ${mark}%`);
  console.log(`Next year you will be ${15 + 1}`);
}

Execution log

Hello Ploy, you scored 82%
Hello Ploy, you scored 82%
Next year you will be 16

You will use backticks for every email body, message and file name you build in this course.

Next: making decisions →

if, else if, else

Code checks conditions from the top and runs the first block that is true, then skips the rest.

Code.gs

function grade() {
  const mark = 68;
  let result;
  if (mark >= 80) {
    result = 'A';
  } else if (mark >= 70) {
    result = 'B';
  } else if (mark >= 60) {
    result = 'C';
  } else {
    result = 'Try again';
  }
  console.log(`Mark ${mark} → ${result}`);
}

Execution log

Mark 68 → C

Change mark to 85, 70, 59 and run each time. Predict the answer first.

The short form

For a simple either/or, the ternary operator fits on one line: condition ? valueIfTrue : valueIfFalse.

const status = mark >= 50 ? 'Pass' : 'Fail';

= is not ===

if (mark = 80) sets mark to 80 and is always true. To compare, write if (mark === 80). This is one of the most common beginner bugs.

Next: functions →

Functions take inputs and return an output

A function can take parameters (inputs in the brackets) and return a result. Then you can reuse it anywhere. One function, many uses.

Code.gs

// A function with parameters and a return value.
function addVat(price, rate) {
  return price + price * rate;
}

// An arrow function does the same job in one line.
const half = (n) => n / 2;

function testFunctions() {
  const total = addVat(100, 0.07);
  console.log(total);           // 107
  console.log(half(total));     // 53.5
  console.log(addVat(250, 0));  // 250
}

Execution log (run testFunctions)

107
53.5
250

addVat has two parameters. When you call addVat(100, 0.07), price is 100 and rate is 0.07.
return sends the answer back and ends the function.
half is an arrow function, a shorter way to write a small function. You will see these a lot inside loops.

Why can't I just press Run on addVat?

You can, but the editor calls it with no inputs, so price and rate are undefined and the result is NaN ("not a number"). Write a small test function, like testFunctions, that calls your function with real values. Run the test.

Next: practice →

Practice

1. BMI calculator Easy

Write bmi(weightKg, heightM) that returns weight ÷ height². Test it with 52 kg and 1.6 m and print it to one decimal place (hint: .toFixed(1)).

Show a solution

Code.gs

function bmi(weightKg, heightM) {
  return weightKg / (heightM * heightM);
}

function testBmi() {
  const value = bmi(52, 1.6);
  console.log(`BMI is ${value.toFixed(1)}`);  // BMI is 20.3
}

2. Even or odd Easy

Write evenOrOdd(n) that returns the text 'even' or 'odd'. Use % and the ternary operator.

Show a solution

Code.gs

function evenOrOdd(n) {
  return n % 2 === 0 ? 'even' : 'odd';
}

function testEvenOdd() {
  console.log(evenOrOdd(4));   // even
  console.log(evenOrOdd(7));   // odd
}

3. Weather advice Medium

Write weatherAdvice(celsius): 35 or more "Very hot", 28 or more "Hot", 20 or more "Nice day", otherwise "Bring a jacket". Test four temperatures.

Show a solution

Code.gs

function weatherAdvice(celsius) {
  if (celsius >= 35) return 'Very hot: stay inside and drink water';
  if (celsius >= 28) return 'Hot: wear a hat';
  if (celsius >= 20) return 'Nice day';
  return 'Bring a jacket';
}

function testWeather() {
  console.log(weatherAdvice(38));
  console.log(weatherAdvice(30));
  console.log(weatherAdvice(24));
  console.log(weatherAdvice(15));
}

4. Shop discount Medium

A shirt costs 120 baht with 10% off. Write fixMe so it prints You pay 108 baht using a template literal.

Show a solution

Code.gs

function fixMe() {
  const price = 120;
  const discount = 0.1;
  const toPay = price - price * discount;
  console.log(`You pay ${toPay} baht`);  // You pay 108 baht
}

Next: quiz →

Check yourself

1. Which keyword makes a variable you cannot reassign?

2. What does console.log('7' + 3) print?

3. What is the value of 17 % 5?

4. Which is true?

5. What does this print? const n = 3; console.log(`n is ${n * 2}`)

6. A function has no return line. What does calling it give back?