echarts中关于merge的代码

function merge(target, source, overwrite) {
    // We should escapse that source is string
    // and enter for ... in ...
    if (!isObject$1(source) || !isObject$1(target)) {
        return overwrite ? clone(source) : target;
    }

    for (var key in source) {
        if (source.hasOwnProperty(key)) {
            var targetProp = target[key];
            var sourceProp = source[key];

            if (isObject$1(sourceProp)
                && isObject$1(targetProp)
                && !isArray(sourceProp)
                && !isArray(targetProp)
                && !isDom(sourceProp)
                && !isDom(targetProp)
                && !isBuiltInObject(sourceProp)
                && !isBuiltInObject(targetProp)
                && !isPrimitive(sourceProp)
                && !isPrimitive(targetProp)
            ) {
                // 如果需要递归覆盖，就递归调用merge
                merge(targetProp, sourceProp, overwrite);
            }
            else if (overwrite || !(key in target)) {
                // 否则只处理overwrite为true，或者在目标对象中没有此属性的情况
                // NOTE，在 target[key] 不存在的时候也是直接覆盖
                target[key] = clone(source[key], true);
            }
        }
    }

    return target;
}