formatDate (#1)

* add formatDate

* add function formatDate

README.md

@@ -20,6 +20,7 @@ JavaScript implementation of formulas for humans.
     - [pi](#pi)
   - [Functions](#functions)
     - [concat](#concat)
+    - [formatDate](#formatDate)
     - [join](#join)
     - [now](#now)
     - [toNumber](#toNumber)
@@ -111,6 +112,27 @@ concat(...text)
 concat('Hello', 'World') // HelloWorld
 ```
 
+### formatDate
+
+Format a date using format string.
+
+<b>Types</b>
+
+Input: `string`<br>
+Output: `string`
+
+<b>Syntax</b>
+
+```js
+formatDate(date, format)
+```
+
+<b>Examples</b>
+
+```js
+formatDate(now(), 'YYYY-MM-DD') // 1999-11-06
+```
+
 ### join
 
 Inserts the first argument between the rest and returns their concatenation.

src/constants.ts

@@ -1,4 +1,21 @@
-export const MONTHS_NAMES: string[] = [
+export const DAY_NAME: string[] = [
+  'Sun',
+  'Mon',
+  'Tue',
+  'Wed',
+  'Thu',
+  'Fri',
+  'Sat',
+  'Sunday',
+  'Monday',
+  'Tuesday',
+  'Wednesday',
+  'Thursday',
+  'Friday',
+  'Saturday',
+];
+
+export const MONTH_NAME: string[] = [
   'Jan',
   'Feb',
   'Mar',
@@ -11,4 +28,16 @@ export const MONTHS_NAMES: string[] = [
   'Oct',
   'Nov',
   'Dec',
+  'January',
+  'February',
+  'March',
+  'April',
+  'May',
+  'June',
+  'July',
+  'August',
+  'September',
+  'October',
+  'November',
+  'December',
 ];

src/index.test.ts

@@ -1,5 +1,5 @@
-import { concat, e, join, now, pi, toNumber } from '.';
-import { MONTHS_NAMES } from './constants';
+import { concat, e, formatDate, join, now, pi, toNumber } from '.';
+import { MONTH_NAME } from './constants';
 
 describe('CONSTANTS', () => {
   test('e', () => {
@@ -14,13 +14,16 @@ describe('FUNCTIONS', () => {
   test('concat', () => {
     expect(concat('text', 1, 'example')).toBe('text1example');
   });
+  test('formatDate', () => {
+    expect(formatDate(now(), 'MMMM DDDYYYY, HH:mm')).toBe('text1example');
+  });
   test('join', () => {
     expect(join('/', 'text', 1, 'example')).toBe('text/1/example');
   });
   test('now', () => {
     const date = new Date();
     const day = date.getDate();
-    const month = MONTHS_NAMES[date.getMonth()];
+    const month = MONTH_NAME[date.getMonth()];
     const year = date.getFullYear();
 
     expect(now()).toBe(`${month} ${day}, ${year}`);

src/index.ts

@@ -1,14 +1,62 @@
-import { MONTHS_NAMES } from './constants';
+import { DAY_NAME, MONTH_NAME } from './constants';
+
+// CONSTANTS
 
 /** The ratio of a circle's circumference to its diameter. */
 export const pi: number = Math.PI;
 
 /** The base of the natural logarithm. */
 export const e: number = Math.E;
 
+// FUNCTIONS
+
 /** Concatenates its arguments and returns the result. */
 export const concat: (...input: any) => string = (...input) => input.join('');
 
+/** Format a date using format string. */
+export const formatDate: (input: string, format: string) => string = (
+  input,
+  format
+) => {
+  const date = new Date(input);
+  const pad = (val: string) => {
+    if (val.length < 2) {
+      return `0${val}`;
+    }
+    return val;
+  };
+  const d = date.getDate();
+  const D = date.getDay();
+  const H = date.getHours();
+  const m = date.getMinutes();
+  const M = date.getMonth();
+  const Y = date.getFullYear();
+  const flags: any = {
+    D: d,
+    DD: pad(String(d)),
+    DDD: DAY_NAME[D],
+    DDDD: DAY_NAME[D + 7],
+    H,
+    HH: pad(String(H)),
+    M: M + 1,
+    MM: pad(String(M + 1)),
+    MMM: MONTH_NAME[M],
+    MMMM: MONTH_NAME[M + 12],
+    YY: String(Y).slice(2),
+    YYYY: Y,
+    m,
+    mm: pad(String(m)),
+  };
+
+  return format.replace(/([YMDHm])\w*/g, match => {
+    if (match in flags) {
+      return flags[match];
+    }
+
+    return match;
+  });
+};
+
 /** Inserts the first argument betweenthe rest and returns their concatenation. */
 export const join: (...input: any) => string = (...input) => {
   const value = input[0];
@@ -20,7 +68,7 @@ export const join: (...input: any) => string = (...input) => {
 export const now: () => string = () => {
   const date = new Date();
   const day = date.getDate();
-  const month = MONTHS_NAMES[date.getMonth()];
+  const month = MONTH_NAME[date.getMonth()];
   const year = date.getFullYear();
 
   return `${month} ${day}, ${year}`;