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TypeScriptの型について - 基本

投稿日：2020年11月15日

TypeScriptは型付き言語であるため、型への理解が非常に重要になります。この記事ではTypeScriptの中の基本の型について解説しています。

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各型について

Any型

function hello(greeting: any){
    console.log(greeting.greet());
}

hello("Hello World");

$ tsc any_sample.ts

$ node any_sample.js
/home/ogihara/ts_sample/any_sample.js:2
    console.log(greeting.greet());
                         ^

TypeError: greeting.greet is not a function
    at hello (/home/ogihara/ts_sample/any_sample.js:2:26)
    at Object.<anonymous> (/home/ogihara/ts_sample/any_sample.js:4:1)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)

Unkwoun型

function helloWorld(greeting: unknown){　
    console.log(greeting.trim());
}


helloWorld("   Hello World    ");
helloWorld(10);

$ tsc unknown.ts 
unknown.ts:2:26 - error TS2339: Property 'trim' does not exist on type 'unknown'.

2     console.log(greeting.trim());
                           ~~~~
Found 1 error.

function helloWorld(greeting: unknown){　
    if(typeof greeting == "string"){     // タイプを特定しないと使用できない
        console.log(greeting.trim());
    }
}


helloWorld("   Hello World    ");
helloWorld(10);

$ tsc unknown.ts
$ node unknown.js 
Hello World

Boolean型

function checkBoolean(value: boolean){
    if(value == true){   // 値を比較
        console.log("This value is true");
    }else{
        console.log("This value is false");
    }
}

let value: boolean = true;
checkBoolean(value);
checkBoolean(!value); // 値の否定を渡す

$ tsc boolean.ts 
$ node boolean.js
This value is true
This value is false

let a: true = true;
let b: false = false;
const c: boolean = true;

let a: true = true;
let b: false = false;
const c: boolean = true;

a = !a;
b = !b;
c = !c;

$ tsc boolean_literal.ts
boolean_literal.ts:6:1 - error TS2322: Type 'boolean' is not assignable to type 'true'.

6 a = !a;
  ~

boolean_literal.ts:7:1 - error TS2322: Type 'true' is not assignable to type 'false'.

7 b = !b;
  ~

boolean_literal.ts:8:1 - error TS2588: Cannot assign to 'c' because it is a constant.

8 c = !c;
  ~


Found 3 errors.

Number型

let a = 1234;
let b: number = 100;
let c = 60.00;

console.log(a + c);

const a = 1234;
let b: 100 = 100;

let a = Number.MAX_SAFE_INTEGER;
console.log(a);
a = a + 2;
console.log(a);

$ tsc number_sample.ts
$ node number_sample.js
9007199254740991
9007199254740992  //+2したのに？？

String型

let a = "hello"
let b: string = "my name is marsquai"

console.log(a + b);

$ tsc string_sample.ts 
$ node string_sample.js
hellomy name is marsquai

let a: "hello" = "hello"
const b: string = "my name is marsquai"

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さいごに

参考URL

参考書籍

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