javascript 練習Node.js

let変数

nodetest.js

let name = "aaaaa";
name = "bbbbb";

console.log(`name = ${name}`);

C:\Users\oe\test>node nodetest.js
name = bbbbb

C:\Users\oe\test>

const定数

値を再代入出来ない。

nodetest.js

const name = "aaaaa";
name = "bbbbb";

console.log(`name = ${name}`);

C:\Users\oe\test>node nodetest.js
C:\Users\oe\test\nodetest.js:2
name = "bbbbb";
^

TypeError: Assignment to constant variable.
at Object.<anonymous> (C:\Users\oe\test\nodetest.js:2:6)
at Module._compile (node:internal/modules/cjs/loader:1730:14)
at Object..js (node:internal/modules/cjs/loader:1895:10)
at Module.load (node:internal/modules/cjs/loader:1465:32)
at Function._load (node:internal/modules/cjs/loader:1282:12)
at TracingChannel.traceSync (node:diagnostics_channel:322:14)
at wrapModuleLoad (node:internal/modules/cjs/loader:235:24)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:171:5)
at node:internal/main/run_main_module:36:49

Node.js v22.17.1

C:\Users\oe\test>

配列

表示

nodetest.js

let array = [1,2,3,4,5];
console.log(array[3]);

array.push(6);
array.push(7);

console.log(array[6]);
array.pop();
console.log(array[6]);

C:\Users\oe\test>node nodetest.js
4
7
undefined

C:\Users\oe\test>

forEach

nodetest.js

let sum = 0;
let array = [1,2,3,4,5];

array.forEach(num=>sum += num);
console.log(sum)

C:\Users\oe\test>node nodetest.js
15

C:\Users\oe\test>

従来のfor

nodetest.js

let sum = 0;
let array = [1,2,3,4,5];

for (let i=0;i<array.length;i++){
sum += array[i];
}

console.log(sum);

C:\Users\oe\test>node nodetest.js
15

C:\Users\oe\test>

オブジェクト

構造体みたいなもの？

nodetest.js

const hito = {
name: "matsuzou",
age: 109
};

console.log(hito.name);
console.log(hito["name"]);
console.log(hito.age);

C:\Users\oe\test>node nodetest.js
matsuzou
matsuzou
109

C:\Users\oe\test>

関数

nodetest.js

function add(a, b){
return a + b;
}

console.log(add(5, 6));

C:\Users\oe\test>node nodetest.js
11

C:\Users\oe\test>

入れ子になった関数

nodetest.js

function outer(){
let count=0;

return function inner(){
count++;
return count;
}
}

const counter = outer();
console.log(counter());

C:\Users\oe\test>node nodetest.js
1

C:\Users\oe\test>

非同期

nodetest.js

const promise = new Promise((resolve) => {
setTimeout( ()=> resolve("完了"),5000);
});
promise.then(result=>console.log(result));

C:\Users\oe\test>node nodetest.js
完了

C:\Users\oe\test>

非同期　複数処理

各Promiseのresolveに渡した引数が、results配列の要素になる
1つでも失敗すると、結果配列は生成されない

nodetest.js

const task1 = new Promise( resolve =>
setTimeout( ()=>resolve("処理1完了"),2000 )
);

const task2 = new Promise( resolve =>
setTimeout( ()=>resolve("処理2完了"),1500 )
);

const task3 = new Promise( resolve =>
setTimeout( ()=>resolve("処理3完了"),2500 )
);

const task4 = new Promise( resolve =>
setTimeout( ()=>resolve("処理4完了"),1000 )
);

Promise.all([task1,task2,task3,task4])
.then( results => console.log("全処理終了",results) )
.catch( error => console.error("いずれかの処理が失敗") );

C:\Users\oe\test>node nodetest.js
全処理終了 [ '処理1完了', '処理2完了', '処理3完了', '処理4完了' ]

C:\Users\oe\test>