TypeScript 检查空字符串
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原文地址: http://stackoverflow.com/questions/44688840/
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TypeScript check for empty string
提问by Alexander Mills
Is there a way for TypeScript to statically check for an empty string? Is there a way to statically require a non-empty string to be passed to a function?
TypeScript 有没有办法静态检查空字符串？有没有办法静态地要求将非空字符串传递给函数？
let fn = function(a:string){
};
fn('');
or
或者
let a = '';
fn(a);
Can TS help us here?
TS 能帮到我们吗？
采纳答案by Rob
I believe this is as close as you're going to get only using the typing system (rather than having a 'nonEmptyString' class)
我相信这与您仅使用打字系统（而不是使用“nonEmptyString”类）一样接近
type nonEmptyString = never; // Cannot be implicitly cast to
function isNonEmptyString(str: string): str is nonEmptyString {
return str && str.length > 0; // Or any other logic, removing whitespace, etc.
}
Testing it:
测试它：
let fn = function(a: nonEmptyString) {
}
let someStr = '';
if (isNonEmptyString(someStr)) {
fn(someStr); // Valid
} else {
fn(someStr); // Compile error
}
Unfortunately, you end up with warts since nonEmptyStringis never. Which means you need to explicitly cast nonEmptyStringback to string.
不幸的是，您最终会长疣，因为nonEmptyString是never。这意味着您需要显式转换nonEmptyString回string.
let fn = function(a: nonEmptyString) {
let len = a.length; // Invalid
let len2 = (<string>a).length; // Valid
let str = a + 'something else'; // Valid (str is now typed as string)
}
One possible resolution is:
一种可能的解决方法是：
type nonEmptyString = string & { __nonEmptyStr: never };
Which alleviates the problem of having to explicitly cast back to a string (all three tests above are valid), but does pollute the type with __nonEmptyStr(which will be undefinedif referenced).
这减轻了必须显式转换回字符串的问题（上述所有三个测试都是有效的），但确实会污染类型__nonEmptyStr（undefined如果被引用将是）。
回答by Kiara Grouwstra
You could maybe type it with overloads such as to give a bad return type on ""that will make things error as soon as you use it elsewhere:
您可以使用重载键入它，例如给出错误的返回类型""，一旦您在其他地方使用它就会出错：
type MyFnType = {
(a: "") => never;
(a: string) => whatever;
}
function fn: MyFnType = ...
回答by Don Pérignon
The snipet below controls if the string is empty and even if there is empty space string
下面的 snipet 控制字符串是否为空，即使有空字符串
private checkRequired(text:string){
if(text == null) return false;
let n = text.length;
for (let i = 1; i < n; i++){
if (text[i] !== " "){
return true;
}
}
return false;
}
if(this.checkRequired("bla bla blast")){
... do your stuff
}
