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Variables

JavaScript · Unit 1: First steps

With let you create a variable you can change later.

With const, one that doesn't change.

let age = 17;
age = age + 1;
console.log(age);

Prints

18

To change the value of a let, use = again: the new one replaces the old one.

You can even use the current value to work out the new one. A const can't be changed.

let points = 10;
points = 25;
console.log(points);
points = points + 5;
console.log(points);

Prints

25
30

let b = a copies into b the value that a has at that moment.

After that they're independent: if you change a, b stays the same.

let today = 20;
let yesterday = today;
today = 25;
console.log(yesterday, today);

Prints

20 25

Names take letters and numbers, can't start with a number and take no spaces or dashes.

To join words you stick the next one on with a capital: myAge. That's camelCase.

const myAge = 17;
const totalPrice = 250;
console.log(myAge, totalPrice);

Prints

17 250

Every instruction ends with a semicolon.

JavaScript almost always puts it in for you if you forget, but writing it avoids surprises.

const name = "Chloe";
console.log(name);

Prints

Chloe

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

  1. 1. Predict the output

    What does this code print?

    const fruit = "apple";
console.log("fruit");
console.log(fruit);

  2. 2. Complete the code

    Fill in to create the variable the greeting uses.

    const ___ = "Owen";
console.log("Hello,", name);

  3. 3. Put the lines in order

    Order the lines to say hello to Emma.

  4. 4. Predict the output

    What does this code print?

    let weather = "sunny";
console.log("Today:", weather);
weather = "rainy";

  5. 5. Complete the code

    Noah won 5 more points. Fill in so his score goes up.

    let points = 20;
points = ___ + 5;
console.log(points);

  6. 6. Find the bug

    This program should subtract the discount from the price. Which line has the error?

    const price = 100;
const discount = 20;
const total = price + discount;
console.log("Total to pay:", total);

  7. 7. Multiple choice

    Which word do you use to declare a variable you are NOT going to reassign?

  8. 8. Predict the output

    What does this code print?

    let x = 5;
let y = x;
x = 10;
console.log(y);

  9. 9. Find the bug

    This program should print Sophie's final grade. Which line has the error?

    const final grade = 85;
console.log("Grade:", finalGrade);

  10. 10. Multiple choice

    What happens if you forget the semicolon at the end of a line?

  11. 11. Predict the output

    Follow each variable line by line. What does it print?

    let mug = "tea";
const glass = "coffee";
mug = glass;
console.log(mug, glass);
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