【Declare a Read-Only Variable with the const Keyword】フリーコードキャンプを日本語で解説！

おいーす、もりたけです。

フリーコードキャンプを日本語で解説！の第112弾です。

JavaScript Algorithms and Data Structures Certification (300 hours)の中の、

Introduction to the ES6 Challengesの中の、

Declare a Read-Only Variable with the const Keyword

です。

さっそく見ていきましょう。

まずは本文から。

The keyword let is not the only new way to declare variables. In ES6, you can also declare variables using the const keyword.

const has all the awesome features that let has, with the added bonus that variables declared using const are read-only. They are a constant value, which means that once a variable is assigned with const, it cannot be reassigned.

“use strict”;

const FAV_PET = “Cats”;

FAV_PET = “Dogs”; // returns error

As you can see, trying to reassign a variable declared with const will throw an error. You should always name variables you don’t want to reassign using the const keyword. This helps when you accidentally attempt to reassign a variable that is meant to stay constant. A common practice when naming constants is to use all uppercase letters, with words separated by an underscore.

Note: It is common for developers to use uppercase variable identifiers for immutable values and lowercase or camelCase for mutable values (objects and arrays). In a later challenge you will see an example of a lowercase variable identifier being used for an array.

 

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Change the code so that all variables are declared using let or const. Use let when you want the variable to change, and const when you want the variable to remain constant. Also, rename variables declared with const to conform to common practices, meaning constants should be in all caps.

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解説していきます。

今回は変数を宣言する新たなワード、constについてです。

ES6からはletだけでなくconstというものを使って新たに変数を宣言できるようになりました。

constはletとが持つすべての機能もち、さらに再割り当てできないという機能ももっています。

という事で今回はconstについて学びました。

では次回！