是否有标准函数来检查JavaScript中的null，undefined或blank变量？ [英] Is there a standard function to check for null, undefined, or blank variables in JavaScript?
问题描述
是否存在通用JavaScript函数，用于检查变量是否具有值并确保它不是 undefined 或 null ？我有这个代码，但我不确定它是否涵盖所有情况：
Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not undefined or null? I've got this code, but I'm not sure if it covers all cases:
function isEmpty(val){
return (val === undefined || val == null || val.length <= 0) ? true : false;
}
推荐答案
你可以检查一下变量是否具有 truthy 值。这意味着
You can just check if the variable has a truthy value or not. That means
if( value ) {
}
将评估为 true 如果值为不：
- null
- undefined
- NaN
- 空字符串（）
- 0
- false
- null
- undefined
- NaN
- empty string ("")
- 0
- false
上面的列表代表了ECMA- / Javascript中所有可能的 falsy 值。在规范中找到 ToBoolean section。
The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.
此外，如果你不知道是否存在变量（这意味着，如果它是声明），你应该检查 typeof 运算符。例如
Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance
if( typeof foo !== 'undefined' ) {
// foo could get resolved and it's defined
}
如果可以确定变量至少声明，你应该直接检查它是否有如上所示的 truthy 值。
If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.
进一步阅读： http://typeofnan.blogspot.com/2011/01/typeof-is -fast.html
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