JSON Schema Input
Tab: Indent | Enter: Auto-Indent
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
TypeScriptv1.0INTERMEDIATE
New Developer Reference Sheet
High-density technical reference sheet with code tabs, annotations, and visual diagrams.
10 min read
Category: Backend
Core Syntax & Foundations
Essential syntax declarations and type evaluations.
Variable & Type Declarations
Declaring constants, mutable variables, and type annotations.
Variable Scope & Mutability Comparison
| Keyword | Scope | Reassignable | Hoisted |
|---|---|---|---|
| const | Block Scope | ❌ No | ❌ No |
| let | Block Scope | ✅ Yes | ❌ No |
| var | Function Scope | ✅ Yes | ✅ Yes |
TYPESCRIPT
// Constant and type definitions
const appVersion: string = "v2.4";
let isProduction: boolean = true;
const port: number = 8080;
type UserRole = "ADMIN" | "ENGINEER" | "GUEST";
Code Annotations:
const appVersion: stringImmutable type declaration with explicit string annotation
typeof appVersion»"string"
💡
Immutability Tip
Prefer const over let to enforce immutable state bindings across concurrent scopes.
variablestypesbasics