JavaScript Numbers and Math

JavaScript has one number type for whole numbers and decimals.

let whole = 42;
let decimal = 3.14;
let big = 1e6;      // 1000000
console.log(whole + decimal);
console.log(big);

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Special values

console.log(10 / 0);
console.log(Number("abc"));
console.log(isNaN("abc"));

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Decimal numbers and rounding

Computers store decimals in binary, so some sums are not exact.

console.log(0.1 + 0.2);
console.log((0.1 + 0.2).toFixed(2));
console.log((3.14159).toFixed(2));

toFixed() returns a string. Use Number() on it if you need a number again. For money, calculate in the smallest unit (paise, cents) as whole numbers.

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The Math object

Method	Result
Math.round(4.6)	5
Math.floor(4.9)	4 (round down)
Math.ceil(4.1)	5 (round up)
Math.max(3, 9, 5)	9
Math.min(3, 9, 5)	3
Math.sqrt(16)	4
Math.pow(2, 3)	8
Math.abs(-7)	7
Math.PI	3.14159…

console.log(Math.round(4.6));
console.log(Math.floor(4.9));
console.log(Math.ceil(4.1));
console.log(Math.max(3, 9, 5));
console.log(Math.sqrt(16));

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Random numbers

Math.random() gives a decimal from 0 up to (but not including) 1. Multiply and round to get whole numbers.

// random whole number from 0 to 9
let n = Math.floor(Math.random() * 10);

// random whole number from 1 to 6, like a dice
let dice = Math.floor(Math.random() * 6) + 1;

The result is different every time you run it.

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Summary

• All numbers are one type.
• Watch out for NaN, Infinity and 0.1 + 0.2.
• Use Math methods for rounding, power, root and random numbers.
• Next: Conditional Statements.

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Exercises & Assignments
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