javascript - 最后定义的原型(prototype)函数总是在对象初始化时运行

Question

给出以下代码(StoriesMap.js)...

// Initalization; Creates Map
var StoriesMap = function StoriesMap( id, location, tileset ) {
    this.map = L.map( id ).setView([ location.lat, location.lon ], location.zoom);

    // Add inital points to map
    this.populate( location );

    // Redraw on map movement
    this.map.on("viewreset", function( event ) {
        var location = {
            lat: event.target._animateToCenter.lat,
            lon: event.target._animateToCenter.lng,
            zoom: event.target._animateToZoom
        };

        this.redraw( location );
    });

    var mapTiles = L.tileLayer( tileset.url, { attribution: tileset.attribution, minZoom: 4, maxZoom: 12 } ).addTo( this.map );
}


// Put some points on the map
StoriesMap.prototype.populate = function populate( location ) {
    var priority = location.zoom - 3;

    if ( typeof points === "object" ) {
        var buffer = [];

        for ( var i in points ) {
            var point = points[i];

            if ( point.priority <= priority ) {
                var circle = L.circle([ point.lat, point.lon ], 100000, {
                    color: '#f03', fillColor: '#f03', fillOpacity: 0.5
                }).addTo( this.map );
            }
        }
    }

}

// Filters map contents
StoriesMap.prototype.filter = function filter( map, location ) {
    console.log( "filter" );
}

// Redraws map points
StoriesMap.prototype.redraw = function redraw( map, location ) {
    console.log ( this.map )

    for ( var i in map._layers ) {
        if ( map._layers[i]._path != undefined ) {
            try {
                map.removeLayer( map._layers[i] );
            }
            catch(e) {
                console.log("problem with " + e + map._layers[i]);
            }
        }
    }
}

StoriesMap.prototype.fake = function fake() {
    console.log("Always Called when object is Initialized");
}

当我运行时(从我的html):

 var map = new Map();

我总是得到:

Always Called when object is Initialized

在我的控制台中。无论出于何种原因，我最后定义的原型(prototype)被视为构造函数，我不确定为什么。我不希望在初始化对象时运行该函数，它是否被假定为构造函数？这种情况以前发生在我身上，我的正常 react 是创建一个名为“fake”的最后一个原型(prototype)函数并将其留空，但是这个问题的正确解决方案是什么？

Answer 1

此代码在某个时刻(可能在某种构建过程或缩小过程中)连接有其他代码。其他代码以括号语句开头:

StoriesMap.prototype.fake = function fake() {
    console.log("Always Called when object is Initialized");
}

// start of other code...
(...);

但是，JavaScript 将其读作:

StoriesMap.prototype.fake = function fake() {
    console.log("Always Called when object is Initialized");
}(...);

因此，StoriesMap.prototype.fake 不等于该函数，而是使用括号表达式作为参数，立即调用该函数。 StoriesMap.prototype.fake 设置为立即调用函数的返回值。

只要在函数表达式末尾添加一个分号，就不会发生这种情况，因为括号内的语句和函数将用分号分隔:

StoriesMap.prototype.fake = function fake() {
    console.log("Always Called when object is Initialized");
};
(...);

JavaScript 的自动分号插入仅在缺少分号会导致违反语言语法时发生。见底部ES 7.9.2与此类似的示例:

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesised expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.