🌟chaining rules

JS ⟩ value ⟩ object ⟩ accessing properties ⟩ chaining rules

all chaining rules are depicted in the following diagrams.

replit：chaining rules

const { log } = console;

// ⭐ test if `x` is nullish
function isNullish(x) {
    return x === undefined || x === null;
}

// ⭐ about ` a . b ?. c . d ?. e `
testAllCases({ logErrorMessage: false });

// ⭐ test all cases
function testAllCases({
    logErrorMessage = false,
} = {}) {

    // counts
    let nullishCount = 0;
    let nonnullishCount = 0;
    let errorCount = 0;

    // all cases: about ` a . b ?. c . d ?. e `
    const cases = [
        null,
        { b: null },
        { b: { c: null } },
        { b: { c: { d: null } } },
        { b: { c: { d: { e: null } } } },
        { b: { c: { d: { e: 'hi' } } } },
    ];

    // iterate over all cases
    for (const [i, a] of cases.entries()) {
        try {
            const value = a.b?.c.d?.e;
            const nullish = isNullish(value);
            nullish ? nullishCount += 1 : nonnullishCount += 1;
            log(`case ${i + 1}: ${value} (${nullish ? 'N' : 'NN'})`);
        } catch (err) {
            log(`case ${i + 1}: ⛔ ${err.name}${logErrorMessage ? `: ${err.message}` : ''}`);
            errorCount += 1;
        }
    }

    // summary
    log(` cases: ${cases.length}  │  N    NN            // N : nullish`);
    log(`───────────┽────────────         // NN: non-nullish`);
    log(`• live: ${nullishCount + nonnullishCount}  │  ${nullishCount}    ${nonnullishCount}`);
    log(`• dead: ${errorCount}  │`);
    
}

// case 1: ⛔ TypeError
// case 2: undefined (N)
// case 3: ⛔ TypeError
// case 4: undefined (N)
// case 5: null (N)
// case 6: hi (NN)

//  cases: 6  │  N    NN            // N : nullish
// ───────────┽────────────         // NN: non-nullish
// • live: 4  │  3    1
// • dead: 2  │

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const { log } = console; // ⭐ test if `x` is nullish function isNullish(x) { return x === undefined || x === null; } // ⭐ about ` a . b ?. c . d ?. e ` testAllCases({ logErrorMessage: false }); // ⭐ test all cases function testAllCases({ logErrorMessage = false, } = {}) { // counts let nullishCount = 0; let nonnullishCount = 0; let errorCount = 0; // all cases: about ` a . b ?. c . d ?. e ` const cases = [ null, { b: null }, { b: { c: null } }, { b: { c: { d: null } } }, { b: { c: { d: { e: null } } } }, { b: { c: { d: { e: 'hi' } } } }, ]; // iterate over all cases for (const [i, a] of cases.entries()) { try { const value = a.b?.c.d?.e; const nullish = isNullish(value); nullish ? nullishCount += 1 : nonnullishCount += 1; log(`case ${i + 1}: ${value} (${nullish ? 'N' : 'NN'})`); } catch (err) { log(`case ${i + 1}: ⛔ ${err.name}${logErrorMessage ? `: ${err.message}` : ''}`); errorCount += 1; } } // summary log(` cases: ${cases.length} │ N NN // N : nullish`); log(`───────────┽──────────── // NN: non-nullish`); log(`• live: ${nullishCount + nonnullishCount} │ ${nullishCount} ${nonnullishCount}`); log(`• dead: ${errorCount} │`); } // case 1: ⛔ TypeError // case 2: undefined (N) // case 3: ⛔ TypeError // case 4: undefined (N) // case 5: null (N) // case 6: hi (NN) // cases: 6 │ N NN // N : nullish // ───────────┽──────────── // NN: non-nullish // • live: 4 │ 3 1 // • dead: 2 │