JavaScript basics for BJS beginners

Learn JavaScript variables, types, conditions, arrays, objects, loops, and functions with small BJS examples you can run in your Telegram bot.

JavaScript is the language you use to write BJS commands. Its variables, conditions, loops, and functions let your bot calculate a result and choose what to say. Bots.Business adds tools such as Bot.sendMessage for sending that result to Telegram.

You do not need previous programming experience. Work through the examples in order, then combine them in a small /quote command at the end.

Set up a practice command

First, create a bot that replies. In the mobile app, open its Commands and create /practice:

• Leave Answer, Keyboard, and Auto retry time in seconds empty.

• Turn Wait for answer off and leave Allowed only for group empty.

• Put the example in the BJS code editor, tap Save, and send /practice in a private Telegram conversation with the running bot.

Set up a practice command · Example 1

Bot.sendMessage("Hello from BJS!");

The reply is Hello from BJS!. Bot.sendMessage is a function supplied by Bots.Business. The parentheses contain what you pass to it; the quotes mark text. The semicolon ends the statement.

For each example below, replace the previous code completely. Do not paste all examples into one command: several reuse the same variable names. You need no extra libraries. The final example has its own command name, /quote.

Variables: give a value a name

A variable gives a value a name so you can use it later in the code.

Variables: give a value a name · Example 2

let points = 2;
points = points + 3;
const label = "Points: ";
Bot.sendMessage(label + points);

The reply is Points: 5.

• let points = 2 creates a variable with the value 2.

• points = points + 3 reads the old value, adds 3, and stores the result.

• const label creates a variable that you cannot assign a different value to later. Use const when you do not need reassignment; use let when you do.

• + adds two numbers, or joins text when a string is involved.

You will also see var in older BJS examples. It declares a variable too, but its scope rules differ; start with const and let in your own code.

Names are case-sensitive: points and Points are different variables. Choose names such as itemPrice or totalPoints. Do not reuse the built-in BJS names user, chat, Bot, User, Api, or params for your own variables.

Text after // is a comment for the reader; JavaScript does not execute it.

Types: text, numbers, and true or false

Type	Example	Use
String	"Hello"	Text, including text received in chat
Number	4 or 2.5	Counts and calculations
Boolean	true or false	A yes/no value
Null	null	An explicit empty value
Undefined	undefined	A value that has not been supplied or assigned

"4" is text; 4 is a number. false is a Boolean; "false" is text.

Types: text, numbers, and true or false · Example 3

const textNumber = "2";
Bot.sendMessage(textNumber + 1);
Bot.sendMessage(String(Number(textNumber) + 1));

The bot sends 21, then 3. Number(...) converts text to a number, and String(...) converts a value to text. Text such as "apple" cannot become an ordinary number: Number("apple") produces NaN, meaning “not a number.” Validate chat input before calculating with it.

Use matching straight quotes, either "text" or 'text'. Inside a string, \n starts a new line. Curly quotation marks copied from formatted text can cause a syntax error.

Calculations

Calculations · Example 4

const itemPrice = 4;
const quantity = 3;
const total = itemPrice * quantity;
Bot.sendMessage("Total: " + total + " points");

The reply is Total: 12 points. Use + to add, - to subtract, * to multiply, and / to divide. Parentheses control order: (2 + 3) * 4 is 20.

These examples use whole points. JavaScript decimal arithmetic can have rounding differences; do not treat this beginner exercise as a payment implementation.

Conditions: choose what happens

if runs a block only when its condition is true. else handles the other case.

Conditions: choose what happens · Example 5

const points = 12;
const required = 10;

if (points >= required) {
  Bot.sendMessage("You have enough points.");
} else {
  Bot.sendMessage("You need more points.");
}

The reply is You have enough points. Change points to 5 and run it again to reach the other branch. Curly braces { ... } group the statements in a block. Use else if between the first branch and else when you need another condition.

Operator	Meaning
===	Equal value and type: 2 === 2 is true; 2 === "2" is false
!==	Different value or type
> or <	Greater than or less than
>= or <=	Greater/less than, or equal
&&	Both conditions must be true
`
!	Reverse a value's true/false interpretation

Use === for equality checks. A single = assigns a value; it does not compare values. For example, quantity >= 1 && quantity <= 5 checks both ends of a range.

Some values also count as false in a condition: 0, "", null, undefined, and NaN. Non-empty text, including "false", counts as true. Prefer an explicit comparison when you need a particular value.

Arrays and objects: group related values

An array is an ordered list. An object holds named fields.

Arrays and objects: group related values · Example 6

const topics = ["Commands", "Buttons", "BJS"];
const product = { name: "Notebook", price: 4 };

Bot.sendMessage(topics[0]);
Bot.sendMessage(product.name + ": " + product.price);

The replies are Commands and Notebook: 4.

• topics[0] reads the first item. Array indexes start at zero, so the third item is topics[2].

• topics.length is the number of items: 3 here. topics.push("Properties") would append another item.

• product.name reads a named field. product.price = 5 would change that field.

• const stops you assigning a different array or object to the variable; it does not freeze the contents.

user.first_name follows the same field-reading idea. BJS supplies that object for executions with a current user; see context and variables before using it in other kinds of command.

Loops: repeat a small amount of work

A for loop can visit every item in an array. Build one reply and send it after the loop:

Loops: repeat a small amount of work · Example 7

const topics = ["Commands", "Buttons", "BJS"];
let reply = "Topics:\n";

for (let i = 0; i < topics.length; i++) {
  reply = reply + (i + 1) + ". " + topics[i] + "\n";
}

Bot.sendMessage(reply);

The reply is:

Loops: repeat a small amount of work · Example 8

Topics:
1. Commands
2. Buttons
3. BJS

The three parts of for mean: start i at zero; keep going while i is less than the length; add one to i after each pass. i++ is a short way to increment it. The i + 1 in the reply makes the visible numbering start at one.

A while loop repeats while a condition stays true:

Loops: repeat a small amount of work · Example 9

let count = 1;
let reply = "";

while (count <= 3) {
  reply = reply + count + " ";
  count++;
}

Bot.sendMessage(reply.trim());

The reply is 1 2 3. trim() removes whitespace from the start and end. Without count++, this loop would never reach its stopping condition.

Keep loops bounded. A loop cannot wait for the user's next Telegram message, and repeatedly sending messages in a loop can flood a chat. Use Wait for answer to collect the next reply, scheduling for later work, or broadcasts for many recipients.

Functions: name a reusable calculation

A function groups code you want to call. Its parameters receive the values you pass in; return gives a result back to the caller.

Functions: name a reusable calculation · Example 10

function calculateTotal(price, quantity) {
  return price * quantity;
}

const total = calculateTotal(4, 3);
Bot.sendMessage("Total: " + total + " points");

The reply is Total: 12 points. Inside the function, price is 4 and quantity is 3. A function definition alone does not run its body: calculateTotal(4, 3) is the call. A function without a returned value produces undefined.

Function parameters are local to that function. let and const variables declared inside an if, loop, or function block are only available within their scope. Declare a variable before using it, and declare it outside a block if later code needs it.

A JavaScript function in this command is different from another Bots.Business command. Bot.runCommand("/other") does not give you a JavaScript return value from /other; use the documented command and callback flow.

Put it together: a /quote command

Create /quote with the same empty metadata fields as /practice, paste the complete example, and tap Save. The user supplies a quantity after the command name, such as /quote 3.

Put it together: a /quote command · Example 11

const input = (params || "").trim();
if (input === "") {
  Bot.sendMessage("Send /quote followed by a quantity from 1 to 5.");
  return;
}

const quantity = Number(input);
if (!Number.isInteger(quantity) || quantity < 1 || quantity > 5) {
  Bot.sendMessage("Choose a whole quantity from 1 to 5.");
  return;
}

const product = { name: "Notebook", price: 4 };

function calculateTotal(price, count) {
  return price * count;
}

const total = calculateTotal(product.price, quantity);
const lines = [
  "Item: " + product.name,
  "Quantity: " + quantity,
  "Total: " + total + " points"
];
Bot.sendMessage(lines.join("\n"));

In BJS, params contains the text after the command name. params || "" uses an empty string when no value is supplied; here || selects a fallback value. Number.isInteger(...) checks that conversion produced a whole number, and the remaining conditions check its range. lines.join("\n") joins the array into one message with line breaks.

Each early return stops this command's BJS before it builds the quote. This example keeps Answer and Keyboard empty; BJS return does not suppress those separate metadata fields.

Send in Telegram	Expected result
/quote 3	Item: Notebook; Quantity: 3; Total: 12 points, on separate lines
/quote 1 or /quote 5	Totals of 4 or 20 points
/quote	Instructions to supply a quantity
/quote apple, /quote 2.5, /quote 0, or /quote 6	A request for a whole quantity from 1 to 5

This command only calculates a quote. It does not save an order, deduct points, or create a payment.

What survives the next message?

Mel’s tip

A local variable belongs to one execution. Running let points = 2 again starts at 2 again; it does not remember the previous run. Other commands do not inherit these local variables or function definitions.

When you need a saved name, balance, or setting, continue with user and bot properties. That guide explains what belongs to one user and what is shared by the bot. For a value needed only by a handler you call next, use structured options instead of an intermediate property.

Practice and troubleshoot

To apply these basics across several commands, follow Data and dialogs: collect a name, validate a quantity and save checklist progress.

Try changing the product name and price, adding an item to the topics array, or making /quote accept quantities up to 10. Update its validation and help messages together, then test a valid value, an invalid value, and missing input.

If something fails, check spelling, matching quotes and braces, = versus ===, and whether a value is text or a number. Use Check in the editor for syntax and the bot's Errors for execution problems. Save changes before testing in Telegram.

An example from a browser tutorial may use document or alert; a Node.js tutorial may use require. These are not BJS message tools. Use Bot.sendMessage to see a result in this lesson and follow BJS execution and limits when moving on to callbacks and integrations.