IEEE 754

Welcome to the world of IEEE 754.

JavaScript

• 0.1 + 0.2 is . You have to use Math.round(), Number.toFixed(), Number.toPrecision() or something like that to avoid this.
• 0.1 + 0.2 - 0.3 is .
• (0.1 + 0.2) - 0.3 is .
• 0.1 + (0.2 - 0.3) is .

PHP

• echo 0.1 + 0.2; is 0.3. Why? Because PHP displays only a few decimal places by default.
• printf("%.17f", 0.1 + 0.2); is 0.30000000000000004.
• echo 0.1 + 0.2 - 0.3; is 5.5511151231258E-17.
• echo (0.1 + 0.2) - 0.3; is 5.5511151231258E-17.
• echo 0.1 + (0.2 - 0.3); is 2.7755575615629E-17.