JavaScript
JavaScript変数

前書き

構文

• var {変数名} [= {値}];

パラメーター

備考

"use strict";

'use strict';

"use strict"; // or 'use strict';
var syntax101 = "var is used when assigning a variable.";
uhOh = "This is an error!";

var myArray = [ "The following is an array", ["I'm an array"] ];

console.log(myArray[1]); // (1) ["I'm an array"]
console.log(myArray[1][0]); // "I'm an array"

var myGraph = [ [0, 0], [5, 10], [3, 12] ]; // useful nested array

console.log(myGraph[0]); // [0, 0]
console.log(myGraph[1][1]); // 10

var myObject = {
    firstObject: {
        myVariable: "This is the first object"
    }
    secondObject: {
        myVariable: "This is the second object"
    }
}

console.log(myObject.firstObject.myVariable); // This is the first object.
console.log(myObject.secondObject); // myVariable: "This is the second object"

var people = {
    john: {
        name: {
            first: "John",
            last: "Doe",
            full: "John Doe"
        },
        knownFor: "placeholder names"
    },
    bill: {
        name: {
            first: "Bill",
            last: "Gates",
            full: "Bill Gates"
        },
        knownFor: "wealth"
    }
}

console.log(people.john.name.first); // John
console.log(people.john.name.full); // John Doe
console.log(people.bill.knownFor); // wealth
console.log(people.bill.name.last); // Gates
console.log(people.bill.name.full); // Bill Gates

変数の定義

var myVariable = "This is a variable!";

これは変数を定義する例です。この変数はASCII文字（ AZ 、 0-9 、 !@#$など）を持つため "文字列"と呼ばれます。

変数の使用

var number1 = 5;
number1 = 3;

ここでは、「number1」という数値を5に設定しました。しかし、2行目では3に変更しました。変数の値を表示するには、コンソールにログを記録するか、 window.alert() ：

console.log(number1); // 3
window.alert(number1); // 3

加算、減算、乗算、除算などを行うには、次のようにします。

number1 = number1 + 5; // 3 + 5 = 8
number1 = number1 - 6; // 8 - 6 = 2
var number2 = number1 * 10; // 2 (times) 10 = 20
var number3 = number2 / number1; // 20 (divided by) 2 = 10;

また、それらを連結する文字列を追加することもできます。例えば：

var myString = "I am a " + "string!"; // "I am a string!"

変数のタイプ

var myInteger = 12; // 32-bit number (from -2,147,483,648 to 2,147,483,647)
var myLong = 9310141419482; // 64-bit number (from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807)
var myFloat = 5.5; // 32-bit floating-point number (decimal)
var myDouble = 9310141419482.22; // 64-bit floating-point number

var myBoolean = true; // 1-bit true/false (0 or 1)
var myBoolean2 = false;

var myNotANumber = NaN;
var NaN_Example = 0/0; // NaN: Division by Zero is not possible

var notDefined; // undefined: we didn't define it to anything yet
window.alert(aRandomVariable); // undefined

var myNull = null; // null
// to be continued...

配列とオブジェクト

var myArray = []; // empty array

配列は変数の集合です。例えば：

var favoriteFruits = ["apple", "orange", "strawberry"];
var carsInParkingLot = ["Toyota", "Ferrari", "Lexus"];
var employees = ["Billy", "Bob", "Joe"];
var primeNumbers = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31];
var randomVariables = [2, "any type works", undefined, null, true, 2.51];

myArray = ["zero", "one", "two"];
window.alert(myArray[0]); // 0 is the first element of an array
                          // in this case, the value would be "zero"
myArray = ["John Doe", "Billy"];
elementNumber = 1;

window.alert(myArray[elementNumber]); // Billy

オブジェクトは値のグループです。配列とは異なり、私たちはそれらより優れた何かをすることができます：

myObject = {};
john = {firstname: "John", lastname: "Doe", fullname: "John Doe"};
billy = {
    firstname: "Billy",
    lastname: undefined
    fullname: "Billy"
};
window.alert(john.fullname); // John Doe
window.alert(billy.firstname); // Billy

配列["John Doe", "Billy"]を呼び出してmyArray[0]を呼び出すのではjohn.fullname 、 billy.fullnameとbilly.fullname呼び出すことができます。