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typescript.md

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3aee11a4 2025-08-15 Pavel Safronov1# Typescript cheatsheet
2
3## Starting
4
5- install
6 - `npm install TypeScript`
7- initialize, creates `tsconfig.json`
8 - `npx tsc --init`
9- build
10 - `npx tsc`
11- run
12 - `npx ts-node index.ts`
13
14## Basics
15
16- variable declaration
17 - `let`, `const`
18 - NOT `var`
19- variable testing
20 - `typeof foo` -> `string`, `number`, `bigint`, `boolean`, `symbol`, `undefined`, `object`, `function`
21 - `instance of Foo` -> `true` if `foo` is an instance of `Foo`
22- null-ish stuff
23 - `??` -> foo ?? 'default' -> returns `foo` it's set, otherwise returns `'default'`
24 - `!` -> `getFoo()!` -> asserts that `getFoo()` will not return `null` or `undefined`
25- basic types
26 - basics: `string`, `number`, `boolean`, `undefined`, `null`, `void`, `any`, `unknown`, `never` (skipped)
27 ```ts
28 const typeString: string = "This is a string";
29 const typeNumber: number = 42;
30 const typeBoolean: boolean = true;
31 const typeUndefined: undefined = undefined;
32 const typeNull: null = null;
33 const typeVoid: void = undefined; // void is often used for functions that do not return a value
34 const typeUnknown: unknown = "This could be anything";
35 ```
36 - arrays: `string[]`, `number[]`, `boolean[]`
37 ```ts
38 const stringArray: string[] = ["apple", "banana", "cherry"];
39 const numberArray: number[] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
40 const booleanArray: boolean[] = [true, false, true];
41
42 let sub: number[] = numberArray.slice(1, 6); // Get a sub-array from index 1 to 5 (not including index 6)
43 sub.push(11); // Add 11 to the end of the array
44 sub = sub.filter(num => num % 2 === 0); // Filter even numbers
45 sub.splice(2, 1, 12); // Replace the third element with 12
46 sub.shift(); // Remove the first element
47 sub = sub.concat([13, 14]); // Concatenate with another array
48 const arraySum = sub.reduce((sum, curr) => sum + curr, 0); // Sum of elements
49 ```
50 - object
51 - `{ id: number; name: string }`
52 - function
53 - `const len = (param: string): number => { return param.length; }`
54 - `const len = (param: string) => param.length;`
55 - tuple
56 - `[string, number]`
57 ```ts
58 const tuple: [string, number] = ["apple", 5];
59 const [fruit, count] = tuple;
60 ```
61 - Date
62 ```ts
63 const now = new Date();
64 const yesterdayGD = new Date(now);
65 yesterdayGD.setDate(now.getDate() - 1);
66 const yesterdayGT = new Date(now.getTime() - 24 * 60 * 60 * 1000);
67 const durationMs = now.getTime() - yesterdayGD.getTime();
68 ```
69 - Map
70 ```ts
71 const map: Map<string, number> = new Map([
72 ["a", 1],
73 ["b", 2],
74 ["c", 3]
75 ]);
76 map.get("a"); // returns 1
77 map.set("d", 4);
78 map.has("b"); // returns true
79 map.delete("c"); // removes key "c"
80 map.clear(); // removes all entries
81 ```
82 - Set
83 ```ts
84 const set: Set<number> = new Set([1, 2, 3]);
85 set.add(4);
86 set.add(2); // Duplicate, will not be added
87 set.delete(1);
88 ```
89 - Regexp
90 ```ts
91 const regex = /p=(\d+),(\d+) v=(-?\d+),(-?\d+)/;
92 const match = regex.exec("p=1,2 v=-3,4")!;
93 const pos = {
94 x: parseInt(match[1]!, 10),
95 y: parseInt(match[2]!, 10)
96 };
97 const vel = {
98 x: parseInt(match[3]!, 10),
99 y: parseInt(match[4]!, 10)
100 };
101 ```
102- interfaces
103 - example, interface
104 ```ts
105 interface Info {
106 id: number;
107 name: string;
108 coords: {
109 x: number;
110 y: number;
111 };
112 expiration?: Date;
113 tags: string[] | null;
114
115 getInfo: () => string;
116 findTag(tag: string): boolean;
117
118 get size(): number;
119 set size(value: number | string);
120 }
121 ```
122 - example, initialization
123 ```ts
124 const info: Info = {
125 id: 1,
126 name: "Info Object",
127 coords: { x: 10, y: 20 },
128 expiration: new Date(),
129 tags: ["tag1", "tag2"],
130
131 getInfo() {
132 return `ID: ${this.id}, Name: ${this.name}, Coords: (${this.coords.x}, ${this.coords.y})`;
133 },
134
135 findTag(tag: string): boolean {
136 return this.tags ? this.tags.includes(tag) : false;
137 },
138
139 get size(): number {
140 return this.tags ? this.tags.length : 0;
141 },
142 set size(value: number | string) {
143 if (typeof value === "number") {
144 this.tags = Array(value).fill("defaultTag");
145 } else if (typeof value === "string") {
146 this.tags = [value];
147 }
148 }
149 };
150 ```
4a4e9c33 2025-08-17 Pavel Safronov151- generics
152 - example
153 ```ts
154 interface DataSource<ResponseType> {
155 fetchData(): ResponseType;
156 }
157 interface DataSource2<ResponseType extends { status: number }> {
158 fetchData(): ResponseType;
159 }
160 ```