JS 常用筆記

Date

plus by senconds

```javascript= var end = new Date(); var endiso = end.toISOString(); end.setSeconds(end.getSeconds() - 5*60); var startiso = end.toISOString(); console.log(startiso); console.log(endiso);

### parse ISO8601
```javascript=
new Date(iso_8601_dateString)

plus by days

```javascript= var dat = new Date(this.valueOf()); // (1) dat.setDate(dat.getDate() + days); // (2)

### data to iso8601
```javascript=
let today = new Date('05 October 2011 14:48 UTC')
console.log(today.toISOString())  // Returns 2011-10-05T14:48:00.000Z

比較先後

```javascript= const lastOdAt ="2021-09-01T09:50:36.017000+00:00"; const lastD = new Date(lastOdAt); const now = new Date();

console.log(lastD<now)

### check 是 vaild 的Date
```javascript=
const lastOdAt ="null";
const lastD = new Date(lastOdAt);
console.log(isNaN(lastD))

Number

MIN MAX Value

Number.MAX_VALUE 
Number.MIN_VALUE 

Number.MIN_SAFE_INTEGER

Array

for...of

```javascript= const array1 = ['a', 'b', 'c'];

for (const element of array1) { console.log(element); }

// expected output: "a" // expected output: "b" // expected output: "c"

### add other array
```javascript=
const array1 = ['a', 'b', 'c'];
const array2 = ['d', 'e', 'f'];
const array3 = array1.concat(array2);
console.log(array3);
// expected output: Array ["a", "b", "c", "d", "e", "f"]

add new one

```javascript= arr.push("Hola");

### find
```javascript=
const array1 = [5, 12, 8, 130, 44];
const found = array1.find(element => element > 10);
console.log(found);
// Expected output: 12

判斷陣列是否包含特定的元素

```javascript= const array1 = [1, 2, 3];

console.log(array1.includes(2)); // expected output: true

const pets = ['cat', 'dog', 'bat'];

console.log(pets.includes('cat')); // expected output: true

console.log(pets.includes('at'));

## JSON

### JSON.stringify()

```javascript=
console.log(JSON.stringify({ x: 5, y: 6 }));
// expected output: "{"x":5,"y":6}"

console.log(JSON.stringify([new Number(3), new String('false'), new Boolean(false)]));
// expected output: "[3,"false",false]"

object

check empty

```javascript= function isEmpty(obj) { return Object.keys(obj).length === 0; }

### foreach key value

```javascript=
const test = {a: 1, b: 2, c: 3};

for (const [key, value] of Object.entries(test)) {
  console.log(key, value);
}

merge

```javascript= var o1 = { a: 1 }; var o2 = { b: 2 }; var o3 = { c: 3 }; var obj = Object.assign(o1, o2, o3); console.log(obj); // { a: 1, b: 2, c: 3 } console.log(o1); // { a: 1, b: 2, c: 3 }, 目標物件本身也被改變。

### clone
```javascript
Object.assign({},o1)

```javascript= o = new Object(); o.prop = 'exists';

o.hasOwnProperty('prop'); // 回傳 true

### 測試屬性是否存在
```javascript=
o = new Object();
o.prop = 'exists';

function changeO() {
  o.newprop = o.prop;
  delete o.prop;
}

o.hasOwnProperty('prop');   // 回傳 true
changeO();
o.hasOwnProperty('prop');   // 回傳 false

String

empty check

1. Simple length check:

if (str && str.length > 0) {
    // not empty
}

2. Using trim() to avoid whitespace-only strings:

if (str && str.trim().length > 0) {
    // not empty, and not just whitespace
}

3. Strict check:

if (typeof str === 'string' && str.trim() !== '') {
    // not empty and confirmed as string type
}

Recommended method (simple, safe):

if (str && str.trim()) {
    // string is not empty and not whitespace-only
}

• str && ensures str isn't null or undefined.

• .trim() removes whitespace before checking the content.

tags: javascript