每个单词的首字母大写使用

Question

I'm doing a class on javascript this semester, and one of this week's exercises was to take a string of words, and capitalize the first letter of each word. I did that using .map() , the code is here :

 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(" ")); 

And it works perfectly fine. However, I was wondering why, when I try with forEach() , I can't make it work. Here is my code :

 let t1 = "hello how are you doing".split(" "); t1.forEach(function(word) { word[0] = word[0].toUpperCase(); }) console.log(t1.join(" ")); 

My understanding of forEach() is that it cycles through every element of the table, much like a simple for loop. So then shouldn't my code take the first letter of each word, and replace it with the same letter, to which toUpperCase() is applied?

edit : I already know how to capitalize the first letter of each word, I was just asking about the different methods

Answer 1

 let word = 'foo'; word[0] = 'F'; console.log(word); 

The modification of word doesn't take because strings are immutable. You can't change a single character inside a string.

For character access using bracket notation, attempting to delete or assign a value to these properties will not succeed. The properties involved are neither writable nor configurable.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String

Answer 2

First, string in JS is immutable.

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

So word[0] = will not have any effect.

Second, even if it was, you are updating a value of an argument variable and not value in array.

 var a = [1, 2, 3]; a.forEach(function(n) { n = n * 2; }); console.log(a) 

As per discussion with @ deceze , this point is not accurate.

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

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_{@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}

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_{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}

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To get the desired effect, you will have to override value in array.

 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(" ")); 

Reference:

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

Answer 3

So then shouldn't my code take the first letter of each word, and replace it with the same letter, to which toUpperCase() is applied?

Because word is a primitive value hence when you set new value to word , it doesn't carry the reference back to the array.

However, if parameter of forEach is not a primitive value, then the original one gets mutated, see the demo below

 var arr = [[1,2,3],[4,5,6],[7,8,9]]; arr.forEach(function(item){ item.push(1); }) console.log(arr);