为什么输出显示未定义，我该如何摆脱它？

Question

I have 3 functions:

1. reverseCharacter --> reversing characters
2. checkType --> checking type String or Number and reversing it
3. newFunction --> runs through the arrays (arrayTest1, arrayTest2 and arrayTest3), reversing and checking the type of the characters

function reverseCharacters(stringToReverse){ return stringToReverse.split('').reverse().join(''); } function checkType(stringToCheck){ if(typeof (stringToCheck) === 'string'){ console.log(reverseCharacters(stringToCheck)); } else if(typeof (stringToCheck) === 'number'){ console.log(reverseCharacters(String(stringToCheck))); } } let arrayTest1 = ['apple', 'potato', 'Capitalized Words'].reverse(); let arrayTest2 = [123, 8897, 42, 1168, 8675309].reverse(); let arrayTest3 = ['hello', 'world', 123, 'orange'].reverse(); function newFunction(x){ for(let i = 0; i <= x.length; i++){ console.log(checkType(x[i])); } } newFunction(arrayTest3); newFunction(arrayTest2); newFunction(arrayTest1);

When I run the code I get undefined after each value:

egnaro
undefined
321
undefined
dlrow
undefined
olleh
undefined
undefined
9035768
undefined
8611
undefined
24
undefined
7988
undefined
321
undefined
undefined
sdroW dezilatipaC
undefined
otatop
undefined
elppa
undefined
undefined

My question is why it says undefined in the output and how can I get rid of it?

Answer 1

Your function has no explicit return statement, therefore it's return value is undefined. You can prove this by adding return "see - no more undefined"; as the last line of the checkType() function.

Or, simply remove the console.log() part of the call to checkType()

 function reverseCharacters(stringToReverse){ return stringToReverse.split('').reverse().join(''); } function checkType(stringToCheck){ if(typeof (stringToCheck) === 'string'){ console.log(reverseCharacters(stringToCheck)); } else if(typeof (stringToCheck) === 'number'){ console.log(reverseCharacters(String(stringToCheck))); } return 'See, no more undefined'; } let arrayTest1 = ['apple', 'potato', 'Capitalized Words'].reverse(); let arrayTest2 = [123, 8897, 42, 1168, 8675309].reverse(); let arrayTest3 = ['hello', 'world', 123, 'orange'].reverse(); function newFunction(x){ for(let i = 0; i <= x.length; i++){ console.log(checkType(x[i])); } } newFunction(arrayTest3); newFunction(arrayTest2); newFunction(arrayTest1);

Answer 2

Just rewrite your newFunction() like this

 function newFunction(x){ for(let i = 0; i <= x.length; i++){ //console.log(checkType(x[i])); checkType(x[i]) } }

The error is happening because you were trying to console.log() value from checkType() . Since checkType() does not have any return value, it just logs reverseCharacters() return values, you are getting undefined printed.