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src/util/shallowEqual.js

const hasOwnProperty = Object.prototype.hasOwnProperty;

function is(x, y) {
    if (x === y) {
        return x !== 0 || y !== 0 || 1 / x === 1 / y;
    }
    return x !== x && y !== y;
}

export default function shallowEqual(objA, objB) {
    if (is(objA, objB)) {
        return true;
    }

    if (typeof objA !== 'object' || objA === null ||
        typeof objB !== 'object' || objB === null) {
        return false;
    }

    const keysA = Object.keys(objA);
    const keysB = Object.keys(objB);

    if (keysA.length !== keysB.length) {
        return false;
    }

    for (let i=0; i<keysA.length; i++) {
        if (!hasOwnProperty.call(objB, keysA[i]) ||
            !is(objA[keysA[i]], objB[keysA[i]])) {
            return false;
        }
    }
    return true;
}