坂本  健一

坂本 健一

1636414620

Javascript でConstとLetの違い

ES6は、JavaScriptで変数を作成する2つの新しい方法をもたらしました。

  1. const
  2. let

ES6以前は、変数を宣言して初期化する方法は1つしかありませんでした。それは、識別子 varを使用する方法です。 varには次の2つのスコープがあります。

  1. グローバルスコープ
  2. 関数スコープ

スコープは、プログラム内で変数と関数にアクセスできる場所を定義します。でJavaScriptのVaRの対聞かせてポスト、我々はより多くのvarがスコープをブロックされているLET異なる方法について議論してきました。

JavaScript constは、再割り当てできない識別子です。letが再割り当て可能な識別子です。再割り当てする必要がない場合は、コードをできるだけ単純にしたい場合があるため、デフォルトでは「let」よりも「const」を選択します。

 

JavaScript const vs let

` const`識別子は、変数が再割り当てされないことを示すシグナルです。`識別子は、そのようなループのカウンタ変数、またはアルゴリズムの値スワップとして、再割り当てされてもよいという信号であります`。

// app.js

let dana = 'Dana Scully'
console.log(dana)
dana = 'Fox mulder'
console.log(dana)

出力

Dana Scully
Fox mulder

この例では、最初にdana 変数を Dana Scully に宣言して初期化し、次にFoxMulderに再割り当てした ことがわかります 。 両方の値をログに記録しましたが、エラーは発生しませんでした。

 

const の場合 、変数を再割り当てすることはできません。

// app.js

const dana = 'Dana Scully'
console.log(dana)
dana = 'Fox mulder'
console.log(dana)

出力

Dana Scully
/Users/krunal/Desktop/code/node-examples/es/app.js:3
dana = 'Fox mulder'
     ^

TypeError: Assignment to constant variable.
    at Object.<anonymous> (/Users/krunal/Desktop/code/node-examples/es/app.js:3:6)

TypeError:定数変数への割り当てを取得しました

これは、値をconst 変数に再割り当てできないことを意味します。

使用しないでください` VARを`生産コードで。シナリオに応じて、常にconst または letの いずれかを使用 してください。

` VARはあなたがJavaScriptで変数を定義するとき、`今最も弱い識別子です。` var`変数は次のようなものにすることができます

  1. 再割り当てされる場合とされない場合があります。
  2. 変数は、関数全体で使用される場合と使用されない場合があります。手段はそれが変わるかどうかを言うことができません。
  3. varは、ブロックまたはループ用です。

したがって、JavaScriptでvar を使用 ず、代わりにconstletを使用することをお勧めします。

JavaScript typeof

Javascript typeofは、未評価のオペランドのタイプを示す文字列を返す組み込み演算子です。JavaScriptでlet`と`` const`で、それは`使用して識別子の存在を確認することはもはや安全ませんtypeof演算`演算子を。

// app.js

function foo () {
  typeof bar
  let bar = ‘baz’
}
foo()

出力

/Users/krunal/Desktop/code/node-examples/es/app.js:3
  let bar = ‘baz’;


SyntaxError: Invalid or unexpected token

結論

変数が将来変更されることがわかっている場合を除いて、常に ` const`を使用する必要があります。constを使用する理由は、この変数を変更してはならないことを、自分自身とコードを読み取らなければならない他の開発者に示しているためです。

変数値が変化し続ける場合は、letを使用する必要があります。

これで、Javascriptのconstとletの比較は終わりです。

リンク: https://appdividend.com/2020/08/28/javascript-const-vs-let-the-complete-guide/

#javascript 

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Javascript でConstとLetの違い

Rahul Jangid

1622207074

What is JavaScript - Stackfindover - Blog

Who invented JavaScript, how it works, as we have given information about Programming language in our previous article ( What is PHP ), but today we will talk about what is JavaScript, why JavaScript is used The Answers to all such questions and much other information about JavaScript, you are going to get here today. Hope this information will work for you.

Who invented JavaScript?

JavaScript language was invented by Brendan Eich in 1995. JavaScript is inspired by Java Programming Language. The first name of JavaScript was Mocha which was named by Marc Andreessen, Marc Andreessen is the founder of Netscape and in the same year Mocha was renamed LiveScript, and later in December 1995, it was renamed JavaScript which is still in trend.

What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

Today almost all websites are using it as web technology, mind is that there is maximum scope in JavaScript in the coming time, so if you want to become a programmer, then you can be very beneficial to learn JavaScript.

JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");
</script>

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */

</script>

Advantages and Disadvantages of JavaScript

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Niraj Kafle

1589255577

The essential JavaScript concepts that you should understand

As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

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Santosh J

1622036598

JavaScript compound assignment operators

JavaScript is unarguablly one of the most common things you’ll learn when you start programming for the web. Here’s a small post on JavaScript compound assignment operators and how we use them.

The compound assignment operators consist of a binary operator and the simple assignment operator.

The binary operators, work with two operands. For example a+b where + is the operator and the a, b are operands. Simple assignment operator is used to assign values to a variable(s).

It’s quite common to modify values stored in variables. To make this process a little quicker, we use compound assignment operators.

They are:

  • +=
  • -+
  • *=
  • /=

You can also check my video tutorial compound assignment operators.

Let’s consider an example. Suppose price = 5 and we want to add ten more to it.

var price = 5;
price = price + 10;

We added ten to price. Look at the repetitive price variable. We could easily use a compound += to reduce this. We do this instead.

price += 5;

Awesome. Isn’t it? What’s the value of price now? Practice and comment below. If you don’t know how to practice check these lessons.

Lets bring down the price by 5 again and display it.
We use console.log command to display what is stored in the variable. It is very help for debugging.
Debugging let’s you find errors or bugs in your code. More on this later.

price -= 5;
console.log(price);

Lets multiply price and show it.

price *=5;
console.log(price);

and finally we will divide it.

price /=5;
console.log(price);

If you have any doubts, comment below.

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