TypeScript from Zero: Variables with let and const

Episode 2 of TypeScript from Zero. In episode 1 we ran our first program. Today: variables. A variable is a name for a value, so the program can remember it. We use let and const, and meet two real errors. All code below is the final code from the video.

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We stay in the same hello-ts project and add a file:

cd hello-ts
touch variables.ts
code .

1. let: a value that can change

let, a name, =, and a value:

let score = 10;
console.log(score);
score = 15;
console.log(score);

node variables.ts

10
15

2. const: a value that stays

For values that should never change, use const (constant):

const playerName = "Ana";
console.log(playerName);

10
15
Ana

3. Changing a const: crash and error

Add a line that assigns to the constant:

playerName = "Ben";

Node stops while running:

TypeError: Assignment to constant variable.
    at Object.<anonymous> (file:///home/mah/hello-ts/variables.ts:7:12)

tsc finds it without running anything:

npx tsc --noEmit variables.ts

variables.ts:7:1 - error TS2588: Cannot assign to 'playerName' because it is a constant.

Rule: use const by default, and let only when the value really has to change. Delete the bad line.

4. TypeScript remembers the type

score started as a number, so TypeScript knows it is a number. Put text into it:

score = "ten";
console.log(score);

Node does not care and prints ten. But tsc complains:

variables.ts:7:1 - error TS2322: Type 'string' is not assignable to type 'number'.

That is the "types" part of TypeScript. Delete those two lines and tsc prints nothing again.

5. Naming rules

• Letters, digits, underscores and $, but never start with a digit.
• Upper and lower case are different names: score is not Score.
• By habit: first word small, next words with a capital (camelCase), like playerName.

Final code

let score = 10;
console.log(score);
score = 15;
console.log(score);
const playerName = "Ana";
console.log(playerName);

Recap

• let for values that change, const for values that stay.
• TypeScript remembers each variable's type (TS2322, TS2588).

Next episode: Numbers and Math.