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フロントエンドカンファレンス東京

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自己紹介 (近況報告)

Go / TSを書いて暮らしてます

肘の骨にヒビが入りました

iwatsurut

とくに、イベントもなく過ごしています。

仕事で、Go で Web システムを作ることになった。想定よりも複雑で、ちょとこまった。

igrep(山本悠滋)

前回のあらすじ

今回の範囲

Object.hasOwn

code:js

const o1 = {

a: 1,

};

console.log(Object.hasOwn(o1, "a"));

console.log(Object.hasOwn(o1, "b"));

console.log("a" in o1);

console.log("b" in o1);

const o2 = {

b: 2,

};

Object.setPrototypeOf(o2, o1);

console.log(Object.hasOwn(o2, "a"));

console.log(Object.hasOwn(o2, "b"));

console.log("a" in o2);

console.log("b" in o2);

/*

true

false

true

false

false // ここがinと違ってtrueにならない！

true

true

true

*/

Object.preventExtensions

code:js

const buffer1 = new ArrayBuffer(8);

const buffer2 = new ArrayBuffer(8, { maxByteLength: 16 });

console.log(buffer1.resizable);

// Expected output: true

console.log(buffer2.resizable);

// Expected output: false

console.log(Object.preventExtensions(new Uint8Array(buffer1)));

console.log(Object.preventExtensions(new Uint8Array(buffer2)));

/*

false

true

Uncaught TypeError: Cannot prevent extensions

at Object.preventExtensions (<anonymous>)

at <anonymous>:11:23

*/

Object.prototype

code:js

"use strict";

console.log(Object.keys(Object.prototype));

Object.prototype.a = 1;

Object.prototype.b = 2;

console.log(Object.keys(Object.prototype));

console.log({}.a, {}.b);

class A {}

console.log(new A().a, new A().b);

console.log(Object.create(null).a, Object.create(null).b);

/*

[]

1 2

1 2

undefined undefined

*/

Object.prototypeへのObject.setPrototypeOfはnullしか通らない

code:js

console.log(Object.getPrototypeOf(Object.prototype));

const o = {

a: 1,

};

console.log(Object.setPrototypeOf(Object.prototype, o)); // NG

console.log(Object.setPrototypeOf(Object.prototype, null)); // OK

Object.prototype.constructor

実は値を差し替え可能

code:js

Object.prototype.constructor

Object.prototype.constructor = Function.prototype.constructor

Object.prototype.constructor

new Object()

{}

(new Object())()

Uncaught TypeError: (intermediate value) is not a function

Object.prototype.hasOwnProperty

code:js

({a:1}).hasOwnProperty("a");

true

({a:1}).hasOwnProperty("b");

false

({a:1}).hasOwnProperty("b");

false

Object.prototype.hasOwnProperty.bind(undefined)("a")

Uncaught TypeError: Cannot convert undefined or null to object

at hasOwnProperty (<anonymous>)

Object.prototype.hasOwnProperty.bind({})("a")

false

Object.prototype.hasOwnProperty.call({}, "a")

false

Object.prototype.hasOwnProperty.call({a:1}, "a")

true

Object.prototype.hasOwnProperty.call(null, "a")

Uncaught TypeError: Cannot convert undefined or null to object

at hasOwnProperty (<anonymous>)

Object.prototype.isPrototypeOf

code:js

const o1 = {};

undefined

const o2 = {};

undefined

const o3 = {};

undefined

Object.setPrototypeOf(o2, o1);

{}

Object.setPrototypeOf(o3, o2);

{}

o1.isPrototypeOf(o3);

true

Object.prototype.toLocaleString

code:js

({}).toLocaleString()

(10000).toLocaleString()

'10,000'

(10000).toString()

'10000'

Object.prototype

code:js

Object.prototype.toString.call(10000)

Object.prototype.toString.call(true)

Object.prototype.toString.call(undefined)

Object.prototype.toString.call(null)

Object.prototype.toString.call([])

Object.prototype.toString.call("")

Object.prototype.toString.call(new Date())

Object.prototype.toString.call(() => {})

Object.prototype.toString.call(1n)

Object.prototype.toString.call(new Uint8Array())

class A {}

undefined

Object.prototype.toString.call(new A())

Object.prototype.toString.call(async () => {})

'AsyncFunction'

undefined

o.toString()