JavaScript

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Points

arrow function

constant scope

constructor override

default export, named export

forEach, find, filter, map method

Type of variable declaration

There are three ways to declare variables in javascript.

var

not recommend

let

変数

const

定数

var

code:js

var hoge_var = 10;

var hoge_var = 100; // redeclarable

hoge_var = 1000; // reassignable

let

code: js

let hoge_let = 20;

let hoge_let = 200; // cannot be redeclared

hoge_let = 2000; // reassignable

const

code:js

const test_const = 30;

const test_const = 300; // cannot be redeclared

test_const = 3000; // cannot be reassigned

scope

global variables

local variables

global variables

Since global variables can be accessed from anywhere, it is easy to cause conflict with other variables.

Therefore, an error may occur or the output may not be performed as expected.

local variables

Variables used only in for statements and functions

Reference