Mutability And Reference VS Privative Types in JavaScript

Mutability && Primitive && Reference Examples

Mutability And Reference VS Privative Types in JavaScript

Mutability && Primitive && Reference Examples

In JavaScript, String values are immutable, which means that they cannot be altered once created.

For example, the following code:

var myStr = "Bob";
myStr[0] = "J";

cannot change the value of myStr to Job, because the contents of myStr cannot be altered. Note that this does not mean that myStr cannot be changed, just that the individual characters of a string literal cannot be changed. The only way to change myStr would be to assign it with a new string, like this:

var myStr = "Bob";
myStr = "Job";

Objects are passed by reference, are mutable, and can be modified by our functions:

function rotateLeft(arr, num) {
    for (let i = 0; i < num; i++) {
        let el = arr.pop();
        arr.unshift(el);
    }
}
let myArr = [1, 2, 3, 4, 5, ];
rotateLeft(myArr, 2);
console.log(myArr);

Strings are passed by value, are immutable, and a new array is constructed and returned, because it cannot be changed in place.

function rotateString(str, num) {
    return str.slice(num) + str.slice(0, num);
}

let str = "foobar";
let ret = rotateString(str, 3);
console.log(str);
console.log(ret);

Dereferencing

Arrays

To dereference an array, use let [var1, var2] syntax.

let arr = ['one', 'two', 'three'];

let [first] = arr;
console.log(first);

Objects

To dereference attributes from an object, use let {} syntax.

	let me = {
	name: "Ian",
	instruments: ['bass', 'synth', 'guitar'],
	siblings: {
	brothers: ['Alistair'],
	sisters: ['Meghan']
	}
	}

	let {
	name,
	instruments: musical_instruments,
	siblings: {
	sisters
	}
	} = me;

	console.log(sisters);

	function printInstruments({
	instruments
	}) {
	console.log(instruments);
	}
	printInstruments(me);

	function printSiblings({
	siblings: {
	sisters,
	brothers
	}
	}) {
	console.log("Sisters", sisters);
	console.log("Brothers", brothers);
	}

	printSiblings(me);

	//rest parameters
	// combines parameters into array

	...parameterName

	splice is an example of where we 've seen this before

	let arr = 'my dog has fleas'.split(' ');
	arr.splice(3, 1, 'trees')

	//spread operator
	// take an arrray and spread them into arguments

	...argContainer

	OR

	// take an object or array and spread their elements/attributes into another object or array

	function multiply(multiplier, ...theArgs) {
	return theArgs.map(function(element) {
	return multiplier * element
	})
	}

	let arr = multiply(2, 1, 2, 3)
	console.log(arr)

	let me = {
	name: "Ian",
	instruments: ['bass', 'synth', 'guitar'],
	siblings: {
	brothers: ['Alistair'],
	sisters: ['Meghan']
	}
	}

	let countryArr = ['USA', 'Canada', 'UK'];
	//me[countries] = countryArr;

	let myCountries = {
	'countries': countryArr
	};

	let newMe = {
	...me,
	...countries
	}

view raw mutability.js hosted with ❤ by GitHub

	// Immutable vs Mutable

	// Strings are IMMUTABLE
	let str = "abc";
	str[ 1 ] = "x";
	// console.log(str); // => 'abc'

	// We can reassign a variable to another string
	beforeStr = str;

	str += "def"; // str = str + 'def'

	// console.log(str) // 'abcdef'

	// console.log(str === beforeStr); // false

	const anotherStr = "abcdef";
	// console.log(str === anotherStr); // true

	// Arrays are MUTABLE
	let arr = [ "a", "b", "c" ];

	let beforeArr = arr;

	arr[ 1 ] = "x";

	console.log( arr ); // ['a', 'x', 'c']
	console.log( beforeArr ); // ['a', 'x', 'c']
	console.log( arr === beforeArr ); // true

	let anotherArr = [ "a", "b", "c" ]; // same contents, but different array
	console.log( arr === anotherArr ); // false

	let dupArr = arr.slice(); // duplicate of the array
	console.log( dupArr ); // ['a', 'b', c']
	console.log( arr === dupArr ); // false

	let str = "abc";

	let upperCase = str.toUpperCase();

	console.log( str );
	console.log( upperCase );
	console.log( str === upperCase );

	console.log( "abc".toUpperCase() );

view raw mut2.js hosted with ❤ by GitHub

Primitive Data Types in Depth

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