javascript - 使用大写每个单词的第一个字母

Question

这学期我正在上一门关于 javascript 的类(class)，本周的练习之一是取一串单词，并将每个单词的第一个字母大写。我使用 .map() 做到了这一点，代码在这里:

let t1 = "hello how are you doing".split(" ");

let t2 = t1.map(function(word) {
  return word[0].toUpperCase() + word.slice(1);
});
console.log(t2.join(" "));

而且它工作得很好。但是，我想知道为什么当我尝试使用 forEach() 时，我无法使其工作。这是我的代码:

let t1 = "hello how are you doing".split(" ");


t1.forEach(function(word) {
  word[0] = word[0].toUpperCase();
})
console.log(t1.join(" "));

我对 forEach() 的理解是它循环遍历表中的每个元素，很像一个简单的 for 循环。那么我的代码不应该采用每个单词的第一个字母，并将其替换为应用 toUpperCase() 的相同字母吗？

编辑:我已经知道如何将每个单词的第一个字母大写，我只是询问不同的方法

Answer 1

首先，JS中的字符串是不可变的。

var str = 'hello World';
str[0] = 'H';
console.log(str)

因此word[0] =不会有任何效果。

其次，即使是这样，您也正在更新参数变量的值，而不是数组中的值。

var a = [1, 2, 3];

a.forEach(function(n) {
  n = n * 2;
});

console.log(a)

根据与@ deceze的讨论，这一点并不准确。

_{If a string was mutable, that would have worked just fine. Your "Second" doesn't really apply. – deceze}

---

_{@deceze Objects are assigned using reference, so it will have the effect. Primitive values are assigned using value. So n in my understanding will always be a copy of item in array. Any manipulation relating to it should not affect the item in array. And string being a primitive value, I mentioned it. Also, I can't think of any variable that has property and is of primitive type. If you have any idea please let me know. Would be glad to learn. :-) – Rajesh}

---

_{Sure, you're right about that. That's why I'm saying if it was mutable in the first place… Since it's not, the point is moot either way. – deceze}

<小时/>

要获得所需的效果，您必须覆盖数组中的值。

let t1 = "hello how are you doing".split(" ");


t1.forEach(function(word, index) {
  t1[index] = word[0].toUpperCase() + word.substring(1);
})
console.log(t1.join(" "));

引用:

• Are JavaScript strings immutable? Do I need a "string builder" in JavaScript?