typescript 函数参数默认值和可选参数 |
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每个函数的参数默认都是必要的,哪怕你是传null或者undefined,也必须要主动传一下。 function buildName(firstName: string, lastName: string) { return firstName + " " + lastName; } let result1 = buildName("Bob"); // error, too few parameters Expected 2 arguments, but got 1. let result2 = buildName("Bob", "Adams", "Sr."); // error, too many parameters Expected 2 arguments, but got 3. let result3 = buildName("Bob", "Adams"); // ah, just right但是在JavaScript中,每个参数都是可选的,如果你没有传参数,他们的值就是undefined。 如何在TS中实现这样的效果呢?在参数名后面写一个?,这样这个参数就是可选的了。 注意,可选的参数放在最后,必须的参数放在前面。 function buildName(firstName: string, lastName?: string) { if (lastName) return firstName + " " + lastName; else return firstName; } let result1 = buildName("Bob"); // works correctly now let result2 = buildName("Bob", "Adams", "Sr."); // error, too many parameters Expected 1-2 arguments, but got 3. let result3 = buildName("Bob", "Adams"); // ah, just right也可以给参数一个默认值,当调用者没有传该参数,或者传入了undefined时,这个默认值就发动了。 function buildName(firstName: string, lastName = "Smith") { return firstName + " " + lastName; } let result1 = buildName("Bob"); // works correctly now, returns "Bob Smith" let result2 = buildName("Bob", undefined); // still works, also returns "Bob Smith" let result3 = buildName("Bob", "Adams", "Sr."); // error, too many parameters Expected 1-2 arguments, but got 3. let result4 = buildName("Bob", "Adams"); // ah, just right有默认值的参数如果放在必须参数的后面,那么这样函数的签名和 可选参数就是一样的。 如下两个函数,就共享一个type function buildName(firstName: string, lastName?: string) { // ... } function buildName(firstName: string, lastName = "Smith") { // ... }但是,有默认值的参数也可放置在 必须参数的前面,这样如果要想触发这个参数的默认值,必须要主动的传入undefined才可以。 function buildName(firstName = "Will", lastName: string) { return firstName + " " + lastName; } let result1 = buildName("Bob"); // error, too few parameters Expected 2 arguments, but got 1. let result2 = buildName("Bob", "Adams", "Sr."); // error, too many parameters Expected 2 arguments, but got 3. let result3 = buildName("Bob", "Adams"); // okay and returns "Bob Adams" let result4 = buildName(undefined, "Adams"); // okay and returns "Will Adams"参考 Handbook - Functions |
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