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TypeScript Cheatsheet

TypeScript Cheatsheet

Owned by Luke Fisk-Lennon
Last updated: Feb 04, 2022 by Mun Joon Teo
3 min read

Variables & constants

C

int a; int b = 42; #define C 42

TS

let a: number; let b = 42; // type inference const c = 42; // mostly do this

Printing

C

#include <stdio.h> printf("%d\n", 42); printf("%s\n", "foo");

TS

console.log(42); console.log('foo');

Types

Numbers

C

int a; float b; // ... a = 42; b = 4.2;

TS

let a: number; a = 42; a = 4.2;

Strings

C

char a; char *b; a = 'f'; b = "foo";

TS

let a: string; a = 'f'; a = "f"; a = 'foo'; a = "foo";

C

char *a = "foo"; strlen(a); // 3

TS

let a = 'foo'; a.length; // 3

Template strings

C

#include <stdio.h> int a = 49; printf("I love %d\n", a);

TS

const a = 49; console.log(`I love ${a}`); // not for only printing b = `I love ${a}`;

Booleans

C

#include <stdbool.h> bool a = true;

TS

const a: boolean = true; const b = true; // type inference

Conditionals

C

if (1 + 1 == 2) { // ... }

TS

// bad if (1 + 1 == 2) { // ... } // good (does type check) if (1 + 1 === 2) { // ... }

C

int a = 1; int b = 2; int max = a > b ? a : b; // 2

TS

const a = 1; const b = 2; const max = a > b ? a : b; // 2

Functions

C

int triple(int n) { return n * 3; }

TS

const triple1 = (n: number): number => { return n * 3; } // type inference const triple2 = (n: number) => { return n * 3; } // only for single-line functions const triple3 = (n: number) => n * 3;

More types

Type aliases

C

typedef int MyType; MyType a;

TS

type MyType = number; let a: MyType;

Objects

C

struct Course { char *name; int students; } Course cs1511 = { 'Programming Fundamentals', 893 };

TS

type Course = { name: string; students: number; } // could also use type inference let cs1511: Course = { name: 'Programming Fundamentals', students: 893 }

Arrays

C

int a[3] = {1, 2, 3}; a[42] = 4; // segmentation fault

TS

const a: number[] = [1, 2, 3]; a[42] = 4; a; // [1, 2, 3, 4]

C

int a[3] = {1, 2, 3}; int i = 0; while (i < 3) { int n = a[i]; printf("%d\n", n); i++; }

TS

const a: number[] = [1, 2, 3]; let i = 0; while (i < 3) { const n = a[i]; console.log(n); i++; } // preferred a.forEach((n) => { console.log(n); });

C

int a[3] = {1, 2, 3}; int i = 0; while (i < 3) { a[i] = a[i] * 2; i++; } a; // {2, 4, 6}

TS

const a: number[] = [1, 2, 3]; const a2 = a.map((n) => n * 2); a; // [1, 2, 3] a2; // [2, 4, 6]

Union types

TS

let a: number | string; a = 42; a = 'foo'; let b = 'foo' | 'bar'; b = 'foo'; b = 'bar'; b = 'baz'; // compile error

Undefined & null

TS

let a: number; console.log(a); // undefined let b: number | null; b = 42; b = null; // the "nothing" type

 

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