Operators and Conditions

Introduction

A program that always does the same thing is not very useful. Real programs decide: if the password is wrong, show an error; if the basket is empty, hide the checkout button. To decide, you need to compare values.

Arithmetic

• % is the remainder: what is left after dividing. 7 % 2 is 1. It is handy for checking odd and even numbers.
• ** is "to the power of".

Computers store decimals in binary, which causes small surprises:

Comparison

Comparisons produce a boolean, true or false.

Use === not ==

• === (strict equality) compares value and type.
• == (loose equality) converts types first, which gives odd results.

Always use === and !==. It avoids a whole class of bugs.

if, else if, else

An if statement runs code only when a condition is true.

JavaScript checks each condition in order and runs the first block whose condition is true. Change temperature and run it again.

Logical operators

• && means and: both must be true.
• || means or: at least one must be true.
• ! means not: flips true and false.

Truthy and falsy

In a condition, some values act like false: false, 0, "" (empty string), null, undefined and NaN. Everything else acts like true.

The ternary operator

A short way to choose between two values:

Try it yourself

Write a program that checks a number. If it is even, print "even", otherwise print "odd". Hint: use % and ===.

Summary

• Arithmetic: + - * / % **. Comparison gives true or false.
• Prefer === and !==.
• if, else if, else choose what to run. &&, || and ! combine conditions.
• Values like 0, "", null and undefined are falsy.