🔸next()

method of an iterator that returns an "iteration result"

JS⟩ iteration ⟩ iterator ⟩ next()

a method that returns an iteration result.

🗺️ 圖表

value of yield expression vs. the next() method

⭐️ 重點

the argument to the next() method：

• is the value of the previous yield. ( 👉 see： 💈範例 )

• is ignored in the first call of next() method. (since there's no previous yield yet)

• can be considered as a new starting point for the current next() call. (except for the first call, in which the argument is ignored)

the const statement：

const arg2 = yield 1;     // generator code

has two execution contexts：

 yield 1     // in the context of #1 next() call.
 const arg2  // in the context of #2 next() call.

💈範例

• replit：value of yield expression (2)

const { log } = console;

// generator code
function* ints() {

    // argument to the `next(arg)` method
    // ---------------------------------------------
    // `arg`:
    //    • is the "value" of PREVIOUS "yield expression".
    //    • is ignored in the first call of next().
    //      (since there's NO previous yield expression)
    //    • can be considered a "new starting point" for
    //      the "current" call of next() method.
    //      (except for the first call, in which `arg` is ignored)

                                    // #1 next() call
                                    // ---------------
    //                    ╭─1─╮     //   1. value for #1 call
    const arg2   =  yield   1  ;    //      (execution stops at Y.E.)
    //   ╰─ 2 ─╯   ╰── Y.E. ──╯     //   Y.E. : yield expression
    
                                    // #2 next() call
                                    // --------------
                                    //   2. argument sent in by #2 call
    //                   ╭── 3 ──╮  //   3. value for #2 call
    const arg3   = yield [2, arg2]; //      (execution stops at Y.E.)
    //   ╰─ 4 ─╯   ╰──── Y.E. ───╯  // 

                                    // #3 next() call
                                    // --------------
    //     ╭── 5 ──╮                //   4. argument sent in by #3 call        
    return [3, arg3];               //   5. "done" value for #3 call
                                    //      (the iteration is finished)
}

// table settings
const headers = ['   ', 'value', 'done'];
const n = headers.length;    // number of columns
const [colWidth, pad, ext] = [5, 1, 0];
const line = '-'.repeat(colWidth*n + pad*(n-1) + ext);
log(`    value  done`);
log(line);

// log iteration result
function logResult(r, i) {
    let value = r.value === undefined ? 'x' : String(r.value);
    value = value.padEnd(5, ' ');
    const done = (r.done ? '✅' : '❌').padEnd(4, ' ');
    log(`#${i}:  ${value}  ${done}`);
}

// main
let it = ints(); 

const r1 = it.next('a');    // #1 next() call
logResult(r1, 1);

const r2 = it.next('b');    // #2 next() call
logResult(r2, 2);

const r3 = it.next('c');    // #3 next() call
logResult(r3, 3);

// output:
//
//     value  done
// -----------------
// #1:  1      ❌   
// #2:  2,b    ❌   
// #3:  3,c    ✅ 

👥 相關

• a Generator object is an iterable iterator, so it has this method too.

📘 手冊

• Generator.prototype.next()

• iterator protocol

• yield