using string trim and toLowerCase methods in a single array map method not works as expected

Question

I have a string that I want to do a process to it:

1) split it by '.' and make an array

2) make each word and character to lower case

3) trim the array elements

I know that we can do this by coding each step separately but I want to do this in one line. But it confused me why I can't do this using map(s => s.toLowerCase() && s.trim()) only the last method in the map works?

Any suggestions would be greatly appreciated.

let trapSt = '  I Was Sent = I Sent. To Earth = To Moon     '
trapSt = trapSt.split(".")
only_LowerCase_Works_Here = trapSt.map(s => s.trim() && s.toLowerCase()); // ??? why only s.toLowerCase() works here???!!!!
only_Trim_Works_Here = trapSt.map(s => s.toLowerCase() && s.trim()); // ??? why only s.trim() works here???!!!!

console.log(only_LowerCase_Works_Here); 
console.log(only_Trim_Works_Here);

strings are immutable. You could chain the methods s.toLowerCase().trim() or you could simply use toLowerCase on the original string before doing the split. — adiga
– adiga, Commented Nov 5, 2019 at 20:17

gabriel.hayes · Accepted Answer · 2019-11-05 20:18:24Z

1

Because toLowerCase and trim don't modify the string in place, they return the string after the modification.

The && isn't the same as:

s.toLowerCase().trim()

which is what you want.

Try this:

let trapSt = '  I Was Sent = I Sent. To Earth = To Moon     '
trapSt = trapSt.split(".")
only_LowerCase_Works_Here = trapSt.map(s => s.trim() && s.toLowerCase()); // ??? why only s.toLowerCase() works here???!!!!
only_Trim_Works_Here = trapSt.map(s => s.toLowerCase().trim()); // ??? why only s.trim() works here???!!!!

console.log(only_LowerCase_Works_Here); 
console.log(only_Trim_Works_Here);

answered Nov 5, 2019 at 20:18

gabriel.hayes

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2 Comments

Sara Ree Over a year ago

Thanks for the answer ... plus one... Actually I'm surprised how it worked... I remember toLowerCase().trim() resulted in errors before! that was why I used &&...

gabriel.hayes Over a year ago

Not a problem, and thanks! If the "string" in question is null or undefined, you will get errors. Other than that, any valid string should be able to go through .toLowerCase().trim()

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