🔸default parameter

🚧 under construction -> previous parameters

JS ⟩ object ⟩ function ⟩ parameter ⟩ default parameter

🚧

const a = 3;     // ❗this one is NEVER referenced in h0 ~ h3 below
const obj = {a}; // { a: 3 }

// ---------------------------------------------------------------------------
// ⛔ "default value" 與 "paremeter name" 沒辦法用「同一個名字」❗
// ---------------------------------------------------------------------------
function h0({a} = {a: a}) { return a } // ⛔ ReferenceError: Cannot access 'a' before initialization
function h1({a} = {a}) { return a }    // ⛔ ReferenceError: Cannot access 'a' before initialization
function h2({a} = { }) { return a }    // ❗ a = undefined (by default)
function h3({a}      ) { return a }    // ❗ 
function h4({a=a} = {}) { return a }   // ⛔ ReferenceError: Cannot access 'a' before initialization

// this one is OK.
function g ({a} = {a: 0}) { return a } // ✅

💾 replit: referencing previous parameters

const {log} = console;

const a = 3;     // ❗this one is NEVER referenced in h0 ~ h4 below
const obj = {a}; // { a: 3 }

// ---------------------------------------------------------------------------
// ⛔ "default value" 與 "paremeter name" 沒辦法用「同一個名字」❗
// ---------------------------------------------------------------------------

💾 程式：replit 📗 參考：JS.info

// Clock
class Clock {

    // usage: 
    // • const clock = new Clock()
    // • const clock = new Clock({template: 'h:m:s'})
    // ⭐️ default parameter (by object destructuring)
    constructor({ template = 'h:m:s' } = {}) {
        this.template = template;
    }

    // ex: "16:43:03"
    render() {
        
        let date = new Date();
        
        let [h, m, s] = [
            date.getHours(), date.getMinutes(), date.getSeconds()
        ].map(x => `${x}`.padStart(2, '0'));

        let output = this.template
            .replace('h', h)
            .replace('m', m)
            .replace('s', s);

        console.log(output);
    }

    stop() {
        clearInterval(this.timer);
    }

    start() {
        this.render();
        this.timer = setInterval(() => this.render(), 1000);
    }
}

const clock = new Clock();
// const clock = new Clock({ template: '_:m:s' });
clock.start();

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Last updated 2 years ago

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const a = 3; // ❗this one is NEVER referenced in h0 ~ h3 below const obj = {a}; // { a: 3 } // --------------------------------------------------------------------------- // ⛔ "default value" 與 "paremeter name" 沒辦法用「同一個名字」❗ // --------------------------------------------------------------------------- function h0({a} = {a: a}) { return a } // ⛔ ReferenceError: Cannot access 'a' before initialization function h1({a} = {a}) { return a } // ⛔ ReferenceError: Cannot access 'a' before initialization function h2({a} = { }) { return a } // ❗ a = undefined (by default) function h3({a} ) { return a } // ❗ function h4({a=a} = {}) { return a } // ⛔ ReferenceError: Cannot access 'a' before initialization // this one is OK. function g ({a} = {a: 0}) { return a } // ✅

const {log} = console; const a = 3; // ❗this one is NEVER referenced in h0 ~ h4 below const obj = {a}; // { a: 3 } // --------------------------------------------------------------------------- // ⛔ "default value" 與 "paremeter name" 沒辦法用「同一個名字」❗ // ---------------------------------------------------------------------------

// Clock class Clock { // usage: // • const clock = new Clock() // • const clock = new Clock({template: 'h:m:s'}) // ⭐️ default parameter (by object destructuring) constructor({ template = 'h:m:s' } = {}) { this.template = template; } // ex: "16:43:03" render() { let date = new Date(); let [h, m, s] = [ date.getHours(), date.getMinutes(), date.getSeconds() ].map(x => `${x}`.padStart(2, '0')); let output = this.template .replace('h', h) .replace('m', m) .replace('s', s); console.log(output); } stop() { clearInterval(this.timer); } start() { this.render(); this.timer = setInterval(() => this.render(), 1000); } } const clock = new Clock(); // const clock = new Clock({ template: '_:m:s' }); clock.start();