Destructuring assignment

Link: https://javascript.info/destructuring-assignment

#til

// if second element is not needed
let [firstName, , title] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];
alert( title ); // Consul

#til

// Works on all kinds of iterables. A syntactic sugar for for...of if you may
let [a, b, c] = "abc"; // ["a", "b", "c"]

let [one, two, three] = new Set([1, 2, 3]); // one = 1, two = 2, three = 3

While destructuring, we can also have functions as default values #til

let options = {
  title: "Menu"
};

let {width: w = prompt("width?"), height: h = 200, title} = options;

alert(title);  // Menu
alert(w);      // whatever was entered in the prompt
alert(h);      // 200

#til

let title, width, height;

// error in this line
// JS thinks that the curly braces comprise a *code block*
{title, width, height} = {title: "Menu", width: 200, height: 100};

// works
({title, width, height} = {title: "Menu", width: 200, height: 100});

Date and time

Link: https://javascript.info/date

Only the first two arguments are mandatory

new Date(year, month, date, hours, minutes, seconds, ms)

new Date(milliseconds) is gives you a Date object with time equal to the number of milliseconds which have passed after Jan 1, 1970 UTC+0
- Dates before that have negative millis!
```
let Jan1_1970 = new Date(0) // 01/01/1970 UTC+0
let Dec31_1969 = new Date(-24 * 60 * 60 * 1000); // 31/12/1969 UTC+0
```

Helper methods

Method	Description
getFullYear()	Get the year (4 digits)
getMonth()	Get the month (0 to 11)
getDate()	Get the day of month (1 to 31)
getHours(), getMinutes(), getSeconds(), getMilliseconds()	Get the corresponding time components
getDay()	Get the day of week, from 0 (Sunday) to 6 (Saturday). The first day is always Sunday, in some countries that’s not so, but can’t be changed.
getTime()	Get the number of milliseconds passed from the January 1st of 1970 UTC+0.
getTimezoneOffset()	Get the difference between UTC and the local time zone, in minutes

All these have a getUTC* counterpart as well
All these also have a corresponding setter

The Date objects have an inbuilt autocorrection feature which is resistant to out-of-bound dates, negative days etc.

let now = new Date()

// 120 seconds from now
now.setSeconds(now.getSeconds() + 120)

// 2 days from now
now.setDate(now.getDate() + 2)

// Out of range date addition
let Feb28_2016 = new Date(28, 1, 2016)
Feb28_2016.setDate(Feb28_2016.getDate() + 2)
// Feb28_2016 is Mar 1, 2016

Coercing a Date object to a number (Symbol.toPrimitive) returns the number of milliseconds, same as getTime(). So subtracting two dates gives the difference in milliseconds.
```
const dateA = new Date(2023, 1, 1)
const dateB = new Date(2023, 1, 2)

+dateA // 1675189800000
dateB - dateA // 86400000
```

We can also use Date.now() for benchmarking a piece of code

let startTime = Date.now()
somePerformanceHeavyCode()
let endTime = Date.now()
endTime - startTime // Time taken to run the code snippet

Date.parse() can read date from a string
- The string format should be: YYYY-MM-DDTHH:mm:ss.sssZ, where:
  - T - Used as the delimiter
  - The optional 'Z' part denotes the time zone in the format +-hh:mm. A single letter Z would mean UTC+0
- Shorter variants are accepted. eg. YYYY/MM/DD, YYYY-MM or even YYYY

JSON methods, toJSON [TODO]

Link: https://javascript.info/json

Content Updates

2023-07-30 - init

Data Types (Part III)

Table Of Contents

Destructuring assignment

Date and time

JSON methods, toJSON [TODO]

Content Updates