TypeScript from Zero: Numbers and Math

Episode 3 of TypeScript from Zero: numbers and math. TypeScript has one type for numbers, called number; whole numbers and decimals are both numbers. We use the operators and the Math helpers, meet the famous 0.1 + 0.2 surprise and NaN, and fix a typo with tsc. All code below is the final code from the video.

• Previous: Variables with let and const. All episodes: playlist.

cd hello-ts
touch numbers.ts
code .

1. Plus and minus

const apples = 7;
const people = 3;
console.log(apples + people);
console.log(apples - people);

10
4

2. Times and divide

* multiplies and / divides. Division always gives the exact answer, not a rounded one:

console.log(apples * people);
console.log(apples / people);

21
2.3333333333333335

3. Remainder and power

% gives the remainder (handy to check if a number is even). ** is "to the power of":

console.log(apples % people);
console.log(2 ** 10);

1
1024

4. The Math helpers

console.log(Math.round(apples / people));
console.log(Math.max(4, 9, 2));

2
9

5. 0.1 + 0.2

console.log(0.1 + 0.2);

0.30000000000000004

Computers store decimals in binary, and some decimals do not fit exactly. This is true in every language. For money, count cents as whole numbers.

6. NaN

Number() turns text into a number. For text that is not a number it gives NaN, "Not a Number", which is still of type number:

console.log(Number("abc"));

NaN

Always check results that come from text.

7. A typo in a Math helper

console.log(Math.rond(2.6));

Node prints all the lines before and then crashes:

TypeError: Math.rond is not a function

tsc finds it first and suggests the fix:

npx tsc --noEmit numbers.ts

numbers.ts:13:18 - error TS2551: Property 'rond' does not exist on type 'Math'. Did you mean 'round'?

Correct the name to Math.round(2.6): tsc prints nothing, and the program runs to the end (the last line prints 3).

Final code

const apples = 7;
const people = 3;
console.log(apples + people);
console.log(apples - people);
console.log(apples * people);
console.log(apples / people);
console.log(apples % people);
console.log(2 ** 10);
console.log(Math.round(apples / people));
console.log(Math.max(4, 9, 2));
console.log(0.1 + 0.2);
console.log(Number("abc"));
console.log(Math.round(2.6));

10
4
21
2.3333333333333335
1
1024
2
9
0.30000000000000004
NaN
3

Recap

• One type, number; operators + - * / % **.
• Math.round, Math.max and friends; the decimal surprise; NaN.

Next episode: Strings.