big.js 源码解析：Big 构造函数与加法精度处理机制

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<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <script src='https://cdn.jsdelivr.net/npm/big.js@6.2.1/big.min.js'></script>
</head>
<body>
    <script>
        new Big(0.1);
    </script>
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function _Big_() {
    function Big(n) {
        var x = this;
        // 可以通过函数调用的形式来创建 Big 对象
        if (!(x instanceof Big)) return n === UNDEFINED ? _Big_() : new Big(n);
        // 区分是否为 Big 示例.
        if (n instanceof Big) {
            x.s = n.s;
            x.e = n.e;
            x.c = n.c.slice();
        } else {
            // 边界处理
            if (typeof n !== 'string') {
                if (Big.strict === true && typeof n !== 'bigint') {
                    throw TypeError(INVALID + 'value');
                }
                // Minus zero?
                n = n === 0 && 1 / n < 0 ? '-0' : String(n);
            }
            parse(x, n);
        }
        x.constructor = Big;
    }
    ...
    return Big;
}

P.plus = P.add = function (y) {
    // 1. 用 x 和 Big 两个变量分别保存 this(调用者) 和 Big 构造函数
    var e, k, t,
        x = this,
        Big = x.constructor;
    // 2. 将入参转化为 Big 对象
    y = new Big(y);
    // 3. 判断是否符号不同，如果不同则直接调用 minus 做减法（1 + （-1）=== 1 - 1）
    if (x.s != y.s) {
        y.s = -y.s;
        return x.minus(y);
    }
    // 4. 分别存储 x 和 y 各自的小数点位置以及 number 数组
    var xe = x.e,
        xc = x.c,
        ye = y.e,
        yc = y.c;
    // 5. Either zero?
    if (!xc[0] || !yc[0]) {
        if (!yc[0]) {
            if (xc[0]) {
                y = new Big(x);
            } else {
                y.s = x.s;
            }
        }
        return y;
    }
    // 6. copy xc 数组
    xc = xc.slice();
    // 7. 补 0（对齐 x 和 y 的长度）
    // Prepend zeros to equalise exponents.
    // Note: reverse faster than unshifts.
    if (e = xe - ye) {
        if (e > 0) {
            ye = xe;
            t = yc;
        } else {
            e = -e;
            t = xc;
        }
        t.reverse();
        for (; e--;) t.push(0);
        t.reverse();
    }
    // 8. 如果 xc 长度大于 yc，则交换它们
    // Point xc to the longer array.
    if (xc.length - yc.length < 0) {
        t = yc;
        yc = xc;
        xc = t;
    }
    e = yc.length;
    // 9. 相加
    // Only start adding at yc.length - 1 as the further digits of xc can be left as they are.
    for (k = 0; e; xc[e] %= 10) k = (xc[--e] = xc[e] + yc[e] + k) / 10 | 0;
    // No need to check for zero, as +x + +y != 0 && -x + -y != 0
    if (k) {
        xc.unshift(k);
        ++ye;
    }
    // Remove trailing zeros.
    for (e = xc.length; xc[--e] === 0;) xc.pop();
    y.c = xc;
    y.e = ye;
    return y;
};

function parse(x, n) {
    ...
    // Determine leading zeros.
    for (i = 0; i < nl && n.charAt(i) == '0';) ++i;
    if (i == nl) {
      // Zero.
      x.c = [x.e = 0];
    } else {
      // Determine trailing zeros.
      for (; nl > 0 && n.charAt(--nl) == '0';);
      x.e = e - i - 1;
      x.c = [];
      // Convert string to array of digits without leading/trailing zeros.
      for (e = 0; i <= nl;) x.c[e++] = +n.charAt(i++);
    }
    ...
}