为什么会有“未定义"的字样?我的字符串开头的文字? [英] Why is there "undefined" text in the beginning of my string?

问题描述

我有一个将AJAX请求的结果连接在一起的函数.

I have a function that concatenates together the results from an AJAX request.

由于某种原因，我的最终字符串以"undefined"开头.

For some reason, my final string starts with "undefined".

这是一个重现该问题的简化示例:

Here is a simplified example that reproduces the problem:

    // In practice, fetched via AJAX from a server
    var vendors = [{ id_vendor: 'V0001' }, { id_vendor: 'V0002' }];

    var row_vendor;

    vendors.forEach(function (value) {
      row_vendor += value.id_vendor;
    });
	
    alert(row_vendor); // undefinedV0001V0002

为什么警报值显示前导的未定义"?

Why does the value alerted display a leading "undefined"?

推荐答案

您没有初始化变量，因此其值为undefined.串联字符串后，将其强制为串联之前的字符串"undefined".

You're not initializing your variable, so its value is undefined. Concatenating a string coerces it to the string "undefined" before the concatenation.

考虑:

var x
alert(x + "test") // undefinedtest

相反，在执行串联之前，请将变量初始化为空字符串:

Instead, initialize your variable to an empty string before performing concatenation:

var x = ""
alert(x + "test") // test

请注意，从功能上讲，首先提取您感兴趣的属性，然后将它们一起join，这样会更清洁:

Note that functionally it is much cleaner to first extract the property you're interested in and then simply join them together:

$.map(vendor, function (v) { return v.vendor_id }).join('')

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