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a7d7463 | 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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 | # JavaScript Skills
## Variables & Types
### Variables
```javascript
// Variables
let name = "JavaScript";
const PI = 3.14159;
var oldStyle = "deprecated";
// Types
typeof name; // "string"
typeof 42; // "number"
typeof true; // "boolean"
```
## Functions
### Regular Function
```javascript
function greet(name) {
return `Hello, ${name}!`;
}
```
### Arrow Function
```javascript
const greet = (name) => `Hello, ${name}!`;
// Multi-line
const multiply = (a, b) => {
return a * b;
};
```
### Callback
```javascript
const processData = (data, callback) => {
const result = doSomething(data);
callback(result);
};
```
## Arrays
### Array Methods
```javascript
const arr = [1, 2, 3, 4, 5];
// map - transform
arr.map(x => x * 2); // [2, 4, 6, 8, 10]
// filter - select
arr.filter(x => x > 2); // [3, 4, 5]
// reduce - accumulate
arr.reduce((sum, x) => sum + x, 0); // 15
// find - search
arr.find(x => x === 3); // 3
// forEach - iterate
arr.forEach(x => console.log(x));
```
## Objects & Destructuring
### Object
```javascript
const person = {
name: "John",
age: 30,
greet() {
return `Hello, I'm ${this.name}`;
}
};
// Access
person.name;
person['age'];
```
### Destructuring
```javascript
const { name, age } = person;
const [first, second] = arr;
```
## Async/Await & Promise
### Promise
```javascript
const fetchData = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("data");
}, 1000);
});
};
```
### Async/Await
```javascript
const getData = async () => {
try {
const response = await fetch(url);
const data = await response.json();
return data;
} catch (error) {
console.error(error);
}
};
```
### Promise.all
```javascript
const [result1, result2] = await Promise.all([
fetch(url1),
fetch(url2)
]);
```
## Common Patterns
### Optional Chaining
```javascript
const value = obj?.nested?.property ?? "default";
```
### Nullish Coalescing
```javascript
const value = input ?? "default value";
```
### Spread Operator
```javascript
const merged = { ...obj1, ...obj2 };
const combined = [...arr1, ...arr2];
```
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